How to manage the CRS of OpenLayers plugin in QGIS?
I am working with OpenLayers plugin in QGIS 1.8 but I am not sure on how should I manage the CRS.
The default CRS for Google/Bing layers is WGS84/Pseudo Mercator (EPGS:3857) but I am not sure on the CRS of the new layer I want to work on.
For the Openlayers plugin, the Project CRS must be in EPSG:3857.
Your other layers can have different Layer CRS, if you have enabled
BTW, current version of QGIS is 2.4, and the openlayers plugin does not support older versions anymore.
You should create new custom CRS and assign it to your layer(s). This quick and dirty approach worked for me after reading all the posts arround here. I use QGIS 2.8.1 and metric system, as I live in Central Europe:
- import Open Layer (in my case, Google Hybrid)
- import your data (in my case, our Municipality Border)
- right click on your data layer and choose "Set Layer CRS"
- Choose "Automatic Coordinate System" - this should bring your data near to desired position (in my case, 470m too much east and 370m too much south)
Measure your x and y difference between specific point on Open Layer and that same point on your layer
Go to Settings, Custom CRS
- Click + sign to add new Custom CRS and give it a name
- Copy parameters from Suggested "Automatc Coordinate system" to new one and alter last digits in +x_0=… and +y_0=… section with values you measure as difference in x and y direction before.
- Click OK to save it and right click on your Layer again - choose "Set Layer CRS".
- First choose previous CRS and click OK, then your new CRS and click OK.
- Your Data should align with Open Layer, otherwise experiment with settings a little more.
Maybe picture is more informative, so I attached one. Hope my approach will work for you as well.
After writing “Towards a template for exploring movement data” last year, I spent a lot of time thinking about how to develop a solid approach for movement data exploration that would help analysts and scientists to better understand their datasets. Finally, my search led me to the excellent paper “A protocol for data exploration to avoid common statistical problems” by Zuur et al. (2010). What they had done for the analysis of common ecological datasets was very close to what I was trying to achieve for movement data. I followed Zuur et al.’s approach of a exploratory data analysis (EDA) protocol and combined it with a typology of movement data quality problems building on Andrienko et al. (2016). Finally, I brought it all together in a Jupyter notebook implementation which you can now find on Github.
There are two options for running the notebook:
- The repo contains a Dockerfile you can use to spin up a container including all necessary datasets and a fitting Python environment.
- Alternatively, you can download the datasets manually and set up the Python environment using the provided environment.yml file.
The dataset contains over 10 million location records. Most visualizations are based on Holoviz Datashader with a sprinkling of MovingPandas for visualizing individual trajectories.
Point density map of 10 million location records, visualized using Datashader
Line density map for detecting gaps in tracks, visualized using Datashader
Example trajectory with strong jitter, visualized using MovingPandas & GeoViews
I hope this reference implementation will provide a starting point for many others who are working with movement data and who want to structure their data exploration workflow.
If you want to dive deeper, here’s the paper:
(If you don’t have institutional access to the journal, the publisher provides 50 free copies using this link. Once those are used up, just leave a comment below and I can email you a copy.)
This post is part of a series. Read more about movement data in GIS.
How to manage the CRS of OpenLayers plugin in QGIS? - Geographic Information Systems
Kartoza releases the CoGo Plugin for QGIS
Kartoza recently published the CoGo Plugin (aka Parcel Plugin) in the QGIS plugin repository. This plugin expands the group of plugins designed to manage SDI (Spatial Data Infrastructure). CoGo ('coordinate geometry') refers to its ability to handle both types of coordinates used in land surveying, namely cartesian coordinates (x,y long/lat) and polar coordinates (bearing and distance).
Kartoza began development of the plugin in 2012 as part of a project commissioned by Spatial Matrix to facilitate the digitising of cadastral property records in Ogun State, Nigeria, where it was (and still is) successfully used in production. In 2016 the plugin was updated by Kartoza and deployed in Niger state, again by Spatial Matrix. In production use, it also helped identify surveying problems eg. land parcels that are overlapping and gaps between land parcels.
Example of Survey Diagram
Why the CoGo plugin?
Cadastral surveying is concerned with the survey and demarcation of land for the purpose of defining parcels of land for registration in a land registry. Any survey that has to be captured in a GIS has to be a true reflection of what occurs on the ground and what is represented by survey diagrams. To do this, we needed to define a tool that allows beacons to be captured and edited, bearings and distances to be defined and then use these to automatically define the land parcels. The plugin supports multiple users working on the same PostGIS database. It facilitates efficient and accurate data capture by operators as well as bulk uploading of structured data.
Features of the plugin
- Supports PostgreSQL 9.6 and higher
- Runs materialised views and their triggers which increases speed for rendering data in QGIS
- A database manager which is linked to the default PostgreSQL database provider in QGIS
- Simple tool to set up the database and associated tables required by the plugin
- A simple user interface: Beacon Manager, Parcels Manager and Bearings and Distance Manager
- A custom topological model where the only simple geometries are beacons, while parcels are defined in views. Beacons defined by bearing and distance strictly honour change
- Sample survey diagram Composer templates for generating official survey diagrams from the now-digital survey data
Example Survey Diagram in map composer.
Much of the core plugin functionality is embedded in the PostgreSQL database. For users who are competent in SQL, it is an advantage as they can do clever SQL to derive more information from the land parcels that have been captured e.g. identifying overlapping parcels or interacting with the parcels in a web front-end. In Ogun State, a localised instance of 1Map was used as a management and QA dashboard, as a public interface to the cadastre and as a tool for facilitating charting at the front desk of the survey office. It ran live on the same database that the operators were working on with QGIS in the back office.
GIS has come a long way in alleviating paper formats for maps, survey diagrams, and this tool will be useful for Surveyors and users involved in SDI. Kartoza will strive to continuously improve the plugin when time and resources are available.
Public datasets in NL
In order to obtain data from public datasets in the Netherlands, the following steps are recommended:
Step 1: Navigate to the area of interest
It can be useful to install the following plugins to navigate to the area of interest:
Unsure of how to install a plugin? > check this tutorial.
Add OpenStreetMap to the project via Web > OpenLayers plugin > OpenStreetMap and use it as a base map to navigate to the area of interest manually. Alternatively, OSM place search can be used to search for a specific location (e.g. Rotterdam). If you cannot see the OSM place search panel after installing the plugin, go to: View > Panels > OSM place search.
After navigating to the area of interest, you may want to create a (spatial) bookmark so that you can easily navigate back to it if you accidentally zoom in or out. A new (spatial) bookmark can be created via: View > New (Spatial) Bookmark.
Note: remember to set the coordinate reference system (CRS) correctly. The same CRS should be used throughout the project. In the Netherlands use:
Step 2: Connect to base data sources
Add the appropriate WFS / ArcGIS feature server connections to your project. Some suggestions for useful datasets are outlined in the tables below. You can also search for other datasets that may be relevant to your project on the PDOK website. When adding the connections:
- Check that the CRS is correct (EPSG:28992 in the Netherlands)
- Make sure to select "Only request features overlapping the view extent"
Unsure of how to add WFS / ArcGIS Feature Server connections? > check this tutorial.
|Feature||Data source||Layer name||Link*|
|Building footprints (+ height)||3D BAG||3D BAG||http://3dbag.bk.tudelft.nl/downloads|
*Please note: the links provided in the table above are not the WFS URLs. Since these datasets are regularly updated, the WFS URL linked to a specific dataset can change over time depending on the latest version. The latest WFS URL for each dataset can be found by clicking on the link provided and selecting the URL of the WFS web-service.
ArcGIS Feature Server connections:
|Feature||Data source||Layer name||URL|
|Vegetation||BGT||begroeidTerreindeel_v||same as above|
|Trees||BGT||vegetatieObject_p||same as above|
Step 3: Save online data for offline use
After adding data from an online server connection, the new layer will be actively connected to the internet. This means that it is not possible to perform further calculations/alterations on the data (will result in QGIS crashes!) and each time the map view changes, the data will be downloaded again. Therefore, the new layer needs to be saved directly to your computer so that the data can be accessed offline. This can be done as follows:
- Right click on the online layer
- Select Export > Save feature as.
- Select the correct settings for your data (file format, file name, CRS)
- Recommended file format for use in QGIS is "GeoPackage"
- In the Netherlands the CRS is EPSG:28992 - Amersfoort / RD New (2D maps) or EPSG:7415 - Amersfoort / RD New (3D maps)
- Make sure to set the extent to "Map Canvas Extent" so that only the required data is saved
In order to obtain data from OpenStreetMap, the following steps are recommended:
Step 1: Install and open the QuickOSM plugin
- Unsure of how to install a plugin? > check this tutorial.
- Open the QuickOSM plugin via Vector > Quick OSM > Quick OSM
Step 2: Enter the required values in the QuickOSM window
- Key = the name of the type of features that should be added to the project (e.g. building, trees, highway, etc)
- Value = [optional] the name of a more specific type of feature (e.g. hotel if the key is building)
- Choose an option from the drop down menu which describes how the features should be located (e.g. in / around / canvas extent)
- Enter the name of the geographic location you would like to obtain features for (e.g. a village, a town. )
Step 3: Run the query
Step 4: Remove unnecessary layers After closing the QuickOSM window, you will notice that the plugin adds a polygon, line and point layer to the project regardless of the type of feature added. Remove any unnecessary layers from the project.
How to manage the CRS of OpenLayers plugin in QGIS? - Geographic Information Systems
MENU BAR provides access to all the main functions and plugins
TOOLBAR provides one-click common functions, and task-specific functions
ADD DATA along the left side is the default location for the Manage Layers Toolbar
LAYER LIST shows all data layers currently added to the project
FILE BROWSER browse to geodata files and drag them onto the map canvas to open them
MAP VIEW provides a dynamic visualization of the active data layers that can be mapped
STATUS BAR provides some vital information about the current project settings
Explore MENU BAR
PROJECT menu is for Opening, Saving, setting Project Properties (such as CRS), launching Print Composer, or saving quick image of map view
EDIT menu is for adding, modifying, deleting spatial features within an editable data layer
VIEW menu is for primary Pan, Zoom, Feature Selection, and Toolbar Controls
LAYER menu is for Adding, Removing, Visibility of data layers, and for changing Layer Projections
SETTINGS menu controls the basic Project settings, Project projection, Language Locale, & other defaults
PLUGINS menu lists the installed Plugins where you can Add or Remove Plugins
VECTOR menu contains Table Manager, Topology Checker, and other useful tools (NOTE: Most of the geoprocessing functions, like Buffer, Point in Polygon, Select by Location, etc have MOVED to the Processing menu)
RASTER menu is for Raster processing functions, such as Heatmap and Zonal Stats (more are enabled with GRASS)
DATABASE menu provides basic import / export operations for databases that have ALREADY been connected using the Manage Layers Add Database buttons.
WEB menu contains the OpenLayers Plugin, where many basemaps can be added (including OSM, Google, Bing, etc), and also has the TileLayer Plugin
PROCESSING menu contains the TOOLBOX for running many geoprocessing tasks, the Model Builder for chaining tasks, and controls the additional scripts that can be run in QGIS such as Grass, Python, R, SAGA, etc.
1.2 Setting-up the environment
1.2.1 Running QGIS for the first time
When you install QGIS, you will get two applications: QGIS Desktop and QGIS Browser. If you are familiar with ArcGIS, QGIS Browser is something similar with ArcCatalog. It is a small application used to preview spatial data and related metadata. For the remainder of this book, we will focus on QGIS Desktop.
By default, QGIS will use the operating system’s default language. To follow the tutorials in this book, I advise you to change the language to English by going to Settings | Options | Locale.
On the first run, the way the toolbars are arranged can hide some buttons. To be able to work efficiently, I suggest that you rearrange the toolbars (for the sake of completeness, I have enabled all toolbars in Toolbars, which is in the View menu). I like to place some toolbars on the left and right screen borders to save vertical screen estate, especially on wide-screen displays.
Additionally, we will activate the file browser by navigating to View | Panels | Browser Panel. It will make a quick access to our spatial data. At the end, the QGIS window on your screen should look similar to the following screenshot:
1.2.2 Introducing the QGIS user interface
Now that we have set up QGIS, let’s get accustomed to the interface. Figure 1.31 shows the QGIS Graphical User Interface (GUI) elements.The biggest area is reserved for the map (Map Display). To the left of the map, there are the Layers and Browser panels. In the image, you can see how the Layers Panel looks once we have loaded some layers. To the left of each layer entry, you can see a preview of the layer style. Additionally, we can use layer group to structure the layer list. The Browser Panel provides us with quick access to our spatial data.
Figure 1.31: QGIS Graphical User Interface
Below the map, we find important information such as (from left to right) the current map Coordinate, map Scale, and the (currently inactive) project coordinate reference system (CRS), for example, EPSG:4326.
Next, there are multiple toolbars to explore. If you arrange them as shown in the previous section, the top row contains the following toolbars:
File: This toolbar contains the tools needed to Create, Open, Save, and Print projects
Map Navigation: This toolbar contains the pan and zoom tools
Attributes: These tools are used to identify, select, open attribute tables, measure, and so on, and looks like this:
The second row contains the following toolbars:
Label: These tools are used to add, configure, and modify labels
Plugins: This currently only contains the Python Console tool, but will be filled in by additional Python plugins
Database: Currently, this toolbar only contains DB Manager, but other database-related tools (for example, the OfflineEditing plugin, which allows us to edit offline and synchronize with databases) will appear here when they are installed
Raster: This toolbar includes histogram stretch, brightness, and contrast control
Vector: This currently only contains the Coordinate Capture tool, but it will be filled in by additional Python plugins
Web: This is currently empty, but it will also be filled in by additional Python plugins
Help: This toolbar points to the option for downloading the user manual and looks like this:
On the left screen border, we place the Manage Layers toolbar. This toolbar contains the tools for adding layers from the vector or raster files, databases, web services, and text files or create new layers:
Finally, on the right screen border, there are two more toolbars:
- Digitizing: The tools in this toolbar enable editing, basic feature creation, and editing
- Advanced Digitizing: This toolbar contains the Undo/Redo option, advanced editing tools, the geometry-simplification tool, and so on, which look like this:
18.104.22.168 Toolbars and panels
Toolbars and panels can be activated and deactivated via the View menu’s Panels and Toolbars entries, as well as by right-clicking on a menu or toolbar, which will open a context menu with all the available toolbars and panels. All the tools on the toolbars can also be accessed via the menu. If you deactivate the Manage Layers Toolbar, for example, you will still be able to add layers using the Layer menu.
As you might have guessed by now, QGIS is highly customizable. You can increase your productivity by assigning shortcuts to the tools you use regularly, which you can do by going to Settings | Configure Shortcuts. Similarly, if you realize that you never use a certain toolbar button or menu entry, you can hide it by going to Settings | Customization. For example, if you don’t have access to an Oracle Spatial database, you might want to hide the associated buttons to remove clutter and save screen estate, as shown in the following screenshot:
22.214.171.124 Projection and Coordinate Reference System (CRS)
Projections define how real-world objects on the curved surface of the earth will be flattened and projected on a map-like planar surface. Different data sources are usually created and distributed in different projections, depending on acquisition techniques and the scope of application. To be able to manipulate and analyze them properly in QGIS, it is important to understand how it interprets and manages information about projections.
QGIS supports about 2,700 CRS. They constitute a database, each item of which is described by an ESPG identifier, and a description line in the format of the PROJ.4 projection library. To store and read information about projection, QGIS uses its own format stored in .qpj files. There are two important points to keep in mind while working with projections: a data source projection and project projection—which are not always the same.
When working with spatial data, it is important that a CRS is assigned to the data and the QGIS project. To view the CRS for the QGIS project, click on Project Properties under Project and choose the CRS tab.
It is recommended that all data added to a QGIS project be projected into the same CRS as the QGIS project. However, if this is not possible or convenient, QGIS can project layers on the fly to the project’s CRS.
If you want to quickly search for a CRS, you can enter the EPSG code to quickly filter through the CRS list. An EPSG code refers to a specific CRS stored in the EPSG Geodetic Parameter Dataset online registry which contains numerous global, regional, and local CRS. An example of a commonly used EPSG code is 4326, which refers to WGS 84. The EPSG online registry is available at http://www.epsg-registry.org/.
To enable on-the-fly projection, perform the following steps:
- Click on Project Properties under Project.
- Choose the CRS tab and Enable ‘on the fly’ CRS transformation.
- Set the CRS that you wish to apply to the project and make all layers that are not set to the project’s CRS transform on the fly.
Figure 1.32: Enabling on-the-fly CRS projection
To view the CRS for a layer, perform the following steps:
Open the layer’s properties by either navigating to Layer | Properties or by right-clicking on the layer in the Layers panel.
Choose Properties from the context menu and then choose the General tab.
If the layer’s CRS is not set or is incorrect, click on Specify to open the CRS selector window and select the correct CRS.
To project a layer to a different CRS, perform the following steps:
Right-click on the layer in the Layers panel and then choose Save As from the context menu.
In the Save vector layer as dialog, set the file format and filename, then set CRS to Selected CRS, click on Change to set the target CRS, and save the file.
To create a new CRS or modify an existing CRS, perform the following steps:
Click on Custom CRS under Settings to open the Custom Coordinate Reference System Definition window.
Click on the Add new CRS button to add a new entry to the CRS list.
With the new CRS selected, we can set the name and parameters of the CRS. The CRS properties are set using the PROJ.4 format. To modify an existing CRS, click on Copy existing CRS and select the CRS from which you wish to copy parameters otherwise, enter the parameters manually. Some background on PROJ.4 is provided below. PROJ.4 is another OSGeo (http://osgeo.org) project used by QGIS, and it is similar to OGR and GDAL. This project is for managing coordinate systems and projections. For a detailed user manual for the PROJ.4 format used to specify the CRS parameters in QGIS, download it from http://download.osgeo.org/proj/OF90-284.pdf.
Simple example on how to work properly with Coordinate Systems in ArcMap
A coordinate system is responsible for displaying your data in the right location. Understanding how to properly work with Coordinate Systems can save unnecessary work and reduce the errors you may encounter in the development of the project. Lets see a simple example regarding this matter.
We have two layers that represent the country limit of South Africa. One layer is found in Geographic Coordinate System: GCS_Cape, Datum: D_Cape and the other in Geographic Coordinate System: GCS_WGS_1984, Datum: D_WGS_1984.
When you open a blank map document in ArcMap, if you go in the Table of Contents, to the Data Frame proprieties you will see that no coordinate system is set.
Here you can set a preferred Coordinate System that will affect the data frame proprieties. All the layers you will load after you set a Coordinate System will be projected on the fly to the selected coordinate system.
If you do not set any preferred Coordinate System, when you will load a layer in the map document, the projection of this layer will be set as data frame Coordinate System.
Step 1: Load in ArcMap the South Africa country limit that has as Geographic Coordinate System: GCS_WGS_1984, Datum: D_WGS_1984. Then check the data frame proprieties again. You will see that this one took the Coordinate System of the layer just opened GCS_WGS_1984.
Lets try now to open the other layer mentioned above, the country limit of South Africa but in a different Coordinate System: Geographic Coordinate System: GCS_Cape, Datum: D_Cape.
When loaded a Geographic Coordinate System Warning appear. This means that the layer you want to load has a different Geographic Coordinate System and a different datum.
ArcMap warns you that in order to align this data properly some parameters must be set.
Let ignore this message and click Close.
In the first place the layer seem to overlay perfectly. Lets make a zoom and check it. We will notice that the layers are not properly overlaid. Measuring the distance between one and another we get approximately 50 m of difference.
The next step will consist in removing the last loaded layer the country limit of South Africa but in a different Coordinate System: Geographic Coordinate System: GCS_Cape, Datum: D_Cape.
Try loading the layer one more time but this time we will configure some parameters in the Geographic Coordinate System Warning that will appear.
Convert from GCS_Cape into GCS_WGS_1984. Then you have to chose a proper Geographic Transformation Cape_To_WGS_1984_1.
We can observe that the layers are perfectly overlaid.
Before and after setting the parameters in the Geographic Coordinate System Warning.
- Open up QGIS to create a new QQIS project that you can work in for georeferencing scanned map images (see QGIS installation guide if you do not already have the software installed). Please keep in mind that this guide was developed for QGIS 3.x so if you have QGIS 2.x installed on your computer you will want to upgrade your installation before proceeding to ensure that these instructions match your software version. If you already have QGIS installed but are unsure which version you have on your computer, try opening QGIS and look closely at the splash screen that displays as the software loads - the version number should be prominently indicated. If you already have QGIS open, you can also check your version number by accessing the Help menu at the top of the screen and selecting Check QGIS Version.
- Once you have successfully opened QGIS you will notice that your project appears empty - this is because you do not have any data currently loaded into it. This is fine because we will be selecting the data to be used in the georeferencing process in an upcoming step. Prior to starting the georeferencing process, it is a good idea to select your QGIS project coordinate system as this will make some of the upcoming steps in this process a little easier. To do this, click Project at the top of your QGIS window (Windows) or top of your screen (MacOS) and select Properties from the menu that appears. In the Project Properties window, click on CRS in the left hand column and then select the CRS you would like your project (and your georeferenced layers) to use in the main window panel. WGS 84 (EPSG:4326) is the standard coordinate reference system (CRS) used for georeferencing maps that are part of the PCL maps collection and it is recommended that you also select this CRS unless you have a specific reason for choosing another option. The easiest way to find the WGS 84 (EPSG:4326) coordinate system is to type WGS 84 in the Filter bar near the top of the Coordinate Reference System Selector window and then browse for it in the now narrowed list under Coordinate reference systems of the world (it should appear near the top of this list).
- In order to georeference maps that do not have identified points with listed coordinate values, it will be necessary to match clearly identifiable points in the map (street corner, corner of a building, etc.) to those same features in a map, dataset, or satellite/aerial imagery that is already georeferenced. One of the easiest ways to obtain satellite imagery for anywhere on Earth that you can use for this method of georeferencing is to the load Google satellite imagery service into QGIS. To do this you must first install the QuickMapServices plugin. This plugin can be installed by clicking on Plugins > Manage and Install Plugins from the top QGIS menu. Once the plugin manager window appears, click on All in the left hand menu and then search for quickmapservices (all one word) to filter out other plugins from your list. Click the button to install the plugin. After the plugin has finished installing successfully, select Web > QuickMapServices > Settings from the top QGIS menu to pull up the plugin's settings configuration window. In this window, click on the More services tab and then click the Get contributed pack button so that you can use the plugin to access the full range of available map services. Once this has been configured correctly, you should be able to select Web > QuickMapServices > Google > Google Satellite to add a new global satellite imagery layer to your QGIS project. For this practice exercise, we will be georeferencing a historic map of the UT campus so go ahead and also zoom in on the imagery layer until you can see the full campus.
- To start georeferencing click on Raster at the top of your QGIS window (Windows) or top of your screen (MacOS). In the drop down menu that appears, click on Georeferencer&hellip to bring up the Georeference window. If you do not see the Georeferencer&hellip option in this menu, that indicates that the Georeferencer&hellip plugin has not yet been activated. To activate the plugin, click on Plugins at the top of your QGIS window and then select Manage and Install Plugins. in the drop down menu to pull up the plugin manager window which lists all available QGIS plugins. Scroll down the list of plugins until you find Georeferencer GDAL. Make sure the box next to this plugin in the list is checked to activate the plugin and then close the plugin manager window.
- Now, click the Open Raster button at the top left corner of the Georeferencer window and browse to the file system location of the map image file you would like to georeference, then click Open. If you do not have a map image file of your own to practice with, or just want to make following the remaining instructions in this guide as easy as possible, you can use the same 1950 map of the University of Texas at Austin campus map shown in the upcoming screenshots of the georeferencing process. You can download this map from the UT Libraries' PCL Maps Collection at http://legacy.lib.utexas.edu/maps/ut_austin/university_of_texas-1950.jpg, save it to your computer, and then browse to the location of the map image file from the Georeferencer window as described above.
- After selecting the map image you want to georeference, you should now be presented with the Coordinate Reference System Selector window where you should select the WGS 84 (EPSG:4326) coordinate system. The WGS 84 (EPSG:4326) coordinate system is a good option in most cases (and if you are voluntarily georeferencing maps from the PCL Maps collection, it is critical that this coordinate system be selected to ensure standardization). Once you have selected your coordinate system click OK and you should see your map appear in the Georeference window.
- To carry out the georeferencing we will now be selecting prominent features in the historical campus map that should also be clearly visible in the modern satellite imagery. It is a good idea to try and select between four and ten matching reference points (also known as tie points) that are spread out across different parts of your map. This is important to ensure that you map is georeferenced as accurately as possible, since too few reference points or reference points that are all clustered in one area of the map can cause the map to distort during the georeferencing process. For this particular map we will be using the four points shown in the graphic below. Start first with the circular planter area indicated in the top left image - in the Georeference window zoom in to this planter (located near the southwest corner of campus) then click the Add Point button to activate the tool and click on the center of the circular planter to add a tie point to the historical campus map. This action will bring up a pop up window where you should see a button titled From Map Canvas. Click on this From Map Canvas button to be temporarily taken back to your main QGIS document window, and then click on the center of that same circular planter in the satellite image. When you do this you should be taken back to the Enter Map Coordinates window and you should notice that latitude and longitude values have now automatically been added to identify the location of the tie point that you have selected which links the non-georeferenced historic map to the already georeferenced aerial imagery. Click OK to finish establishing this tie point. Repeat this procedure for the other three points indicated in the remaining images below to add a total of four tie points to the map.
- Once all four tie points have been successfully added, good go ahead and click the Save GCP Points As button in your Georeferencer toolbar. This will bring up a pop up window that will prompt you to select a save location and file name for your GCP points export. You should give this file the same name as original ungeoreferenced map image file (make sure to remove the .jpg or other image extension in the default file name suggested by the tool though so that it is not saved with a double extension like file.jpg.points). You should also save this file in the same location that you plan to save your georeferenced map image.
- After saving your GCP points, click the green arrow button for the Start Georeferencing tool which is located in your Georeferencer toolbar. The first time you georeferenced a map, this will bring up the Transformation Settings popup window where you have to select a few important parameters that will determine how the georeferencing process is carried out. In the top portion of this window, set the Transformation Type to Polynomial 1, set the Resampling Method to Cubic, and the Target SRS to WGS 84 (EPSG: 4326). Under Output Settings, click the &hellip button in the Output Raster row to select a location to save the georeferenced raster on your computer. When selecting this save location, makes sure that you keep the original name of the file and then append the following information to the file name _transformationtype_resamplingmethod_compressionmethod_coordinatesystem using underscores to separate each important attribute. Thus a map image file named txu-pclmaps-oclc-6587819-na-32 might be saved as txu-pclmaps-oclc-6587819-na-32_polynomial1_cubic_lzw_wgs84. Also, make sure to select the LZW compression type. Once you have correctly selected your parameters, click the OK button to apply your transformation settings and close your Transformation Settings window. At this point you will need to once again click on the green arrow button to start the georeferencing process with these transformation settings. The georeferencing process should complete in a few seconds and you will then see your newly georeferenced map image appear in your QGIS project canvas.
1 Answer 1
We can implement the drawing function into our QGIS2web map by expanding the
variable in our qgis2web.js file.
Being more precise we need to expand the draw.on('drawend', ) option first
The original code looks like this:
Next, if we wish to have the other color of our drawing than measurement tool, we need to replicate the var measureLayer variable, defining the yellow colour already.
we can call id for instance var drawLayer :
In turn, we are able to keep the measure drawings and our drawings separately.
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Trasformazioni datum predefinite¶
OTF depends on being able to transform data into a ‘default CRS’, and QGIS uses WGS84. For some CRS there are a number of transforms available. QGIS allows you to define the transformation used otherwise QGIS uses a default transformation.
In the CRS tab under Settings ‣ Options you can:
- set QGIS to ask you when it needs define a transformation using Ask for datum transformation when no default is defined
modificare la lista di trasformazioni specificate dall’utente.
QGIS asks which transformation to use by opening a dialogue box displaying PROJ.4 text describing the source and destination transforms. Further information may be found by hovering over a transform. User defaults can be saved by selecting Remember selection.
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