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Georeference Raster Data

This guide explains what the process of georeferencing involves and provides detailed instructions for georeferencing raster data using QGIS software.

Georeference By Map Corners

Georeference By Map Corners: Process Overview

This guide provides instructions for georeferencing scanned images of paper maps that have legible geographic coordinate information provided for each of the 4 corners of the map (see example images below). If your map does not coordinate information at the corners of the map extent, you should consult the Georeference By Map Features guide instead as the georeferencing process is significantly different for maps lacking this coordinate information.

This guide describes the georeferencing process in QGIS due to the fact that QGIS is free, open source, compatible with a variety of computer operating systems, and has georeferencing tools that are easy to learn and use (refer to the QGIS installation guide if you do not already have the software installed on your computer). The instructions provided in this guide are highly detailed to ensure that users are able to easily navigate the georeferencing process so you will notice that this guide is longer than most. It will walk you through the following major steps:

  1. Using the Raster > Georeferencer tool to assign ground control points to the four corners of the map extent of a scanned paper map raster image then saving the image with an assigned coordinate system
  2. Using the Raster > Extraction > Clip Raster by Extent tool to crop off the collar of the georeferenced map image and save it as a compressed JPG file

For the full guide instructions, continue reading below.

Instructions

  1. 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.
  2. 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).
  3. 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… to bring up the Georeference window. If you do not see the Georeferencer… option in this menu, that indicates that the Georeferencer… 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.
    Georeferencer plugin activation screenshot
  4. 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 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/ams/africa_1m/txu-pclmaps-oclc-6587819-na-32.jpg, save it to your computer, and then browse to the location of the map image file from the Georeferencer window as described above. As long as you have already successfully selected a coordinate reference system for your project you should be ready to move to the next step. If there were any issues with the coordinate reference system selection you will notice the Coordinate Reference System Selector window appear after you select the map image you want to georeference. If you are presented with this window 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.
  5. Most standardized maps like those that are part of AMS, USGS, or JOG series will have Geographic Coordinate System (GCS) degree values prominently displayed at each of the four corners of the map on the map sheet. This information is extremely useful when georeferencing as it will allow us to determine where each corner of the map is located on Earth so that it can be drawn properly on top of other georeferenced layers in GIS software. To get started generating the Ground Control Points (GCPs) that we will use to georeferenced the map image, zoom in to the top left corner of your map where you should hopefully see GCS coordinates. Then click on the Add GCP button undefined in your Georeferencer window toolbar and click again on the corner when the map extent ends. This will bring up the Enter Map Coordinates window where you can now enter in the coordinate values that you see displayed in the corner you are currently zoomed to. Read the coordinate formatting instructions in this window carefully and make sure that you type the right values in the X and Y fields to ensure that the coordinates are entered correctly. Now repeat this process until you have created GCPs for all four corners of the map.close up of newly added ground control point in map corner 
  6. Double check that the coordinates for the GCP points listed in your GCP table are entered correctly and if everything looks 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.
  7. 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 … 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.tif. 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.
  8. To verify that your map was georeferenced correctly you can add an OpenStreetMap basemap to your QGIS project that you can use to check the positioning of your map image by comparing the locations of prominent features in both maps. To do this, click on XYZ Tiles in your Browser Panel and then drag the OpenStreetMap layer into your Layers panel, taking care to position it below your newly exported georeferenced map image layer.
    Location of OpenStreetMap Layer in QGIS Browser panel
  9. Now that your map has been georeferenced, you may also want to clip your scanned map image to remove the collar (the white marginal space around the map that contains information like the map’s title, scale, legend, etc.) to facilitate use of the raster image in GIS software. To do this 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 Clip Raster by Extent to bring up the tool window.
  10. In the Clip Raster by Extent window that pops up, click the … button next to the Clipping Extent (xmin, xmax, ymin, ymax) field and selectSelect Extent on Canvas from the drop down menu that appears. This will momentarily take you back to the map canvas where you should now click and hold down your cursor on the top left corner of the map extent and then continue holding your mouse button down as you drag your cursor to the bottom right corner of the map extent. You should notice a large red rectangle being drawn over the map as you drag your cursor. Once your cursor is over the bottom right corner of the map, stop holding down your left mouse button to finish your extent selection and return to the Clip Raster by Extent window.
  11. Back in the Clip Raster by Extent window, set your Profile parameter (located under Advanced Parameters) to JPEG compression and adjust the JPEG_QUALITY value to 99. Next, click the … button next to the Converted field to select Save to File… which will allow you to choose where you compressed image will be saved. Next, click on the … button next to the “Clipped (extent)” field to open the Save File window where you should browse to the location of the georeferenced map image that you are trying to compress and click on it to select it. This will automatically populate the name of this image in the File Name field. It is recommended that you modify this file name by adding _c to the end of the file name to denote that this version of the image is compressed. While in this Save File window you will also need to change the Save as Type field value to JPG files (*.jpg) so that the file will be properly compressed. Once you are done entering in these parameters, go ahead and click Run.
  12. Congratulations, you have now successfully georeferenced a scanned map image. If you would like to georeference another map image, go back to your Georeferencer window where you should still see the last image that you processed. To replace this with a new map image that needs georeferencing click the Open Raster at the very left of the toolbar, browse to the location of the new image you want to process, and then click OK. After a few seconds you should see the new map image replace the previous one that you just completed. You can now proceed with georeferencing this new map by starting these instructions again from step 4.

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