Georeferencing and Digitizing Map Images in ArcGIS Pro
Geospatial data is often acquired from imagery that is remotely sensed by airplanes and space satellites. The process of measuring and interpreting images to extract geospatial data is called photogrammetry.
While there are a growing variety of techniques and software packages that can perform photogrammetry to partially automate the capture of two-dimensional and three-dimensional features from images with minimal direct intervention, there are still often occasions where you may need to manually digitize vector features based on contemporary remotely sensed raster imagery or historic paper maps. You can then symbolize these features according to your mapping needs.
This tutorial covers five basic steps for digitizing features from photographic or map images:
- Acquire the Image
- Georeference the Image
- Create a New Feature Class
- Digitize the Features
- Publish the Features
- Save Your Project Package
Acquire the Image
The example image is a historic map of the 1893 Columbian Exposition, commonly referred to as the 1893 Chicago World's Fair. The map was created by cartographer George Cram and published in a commemorative atlas of the fair.
This image was downloaded from the David Rumsey Map Collection. The video below shows how to download and unzip a map from that collection.
If your map contains a border and/or marginalia that you do not want in your map, open the image in Paint and crop only the desired area.
Georeference the Image
While human eyes can interpret an image, most image files do not contain enough information for software to be able to figure out where the image is located on the surface of the earth and map it accordingly.
Using an image as a map layer in ArcGIS Pro requires a step called georeferencing where locations on the map are marked as located at specific latitudes and longitudes. The software then stretches (transforms) the image to align with locations on the ground.
- Create a project with a new map.
- Add Data the image you will be georeferencing. This may take a few seconds depending on the size of the image. The image will appear in the Contents pane, but will not be visible in the correct location because ArcGIS Pro doesn't have clear information on where it is supposed to be located.
- Select the image layer in the Contents pane, select the Imagery tab, and click Georeference.
- Click Set SRS and choose Geographic Coordinate System -> World -> WGS 1984.
- Zoom into the approximate area where the image or map is supposed to be located.
- Click the Fit To Display button. The map image should appear in the center of the screen, although it will not be correctly aligned with the base map.
- Select the Raster -> Appearance tab and turn the transparency up to 50% so you can see the base map under the image.
- Go back to the Georeference tab and under Transformation, select Projective. This tells ArcGIS Pro how to stretch the map, and a projective transformation often works well with historic maps that are already projected.
- Select Add Control Points and add control points by selecting a
location on the image first, then select the matching location on the base map.
- Try to match locations in the extreme four corners first.
- Zoom in as close as possible so your control points are as accurate as possible.
- When trying to align street intersections, aim for the centers of the intersections.
- Look around the map to make sure things align as expected. Add additional points as needed to align the map as closely as possible to the base map.
- If you make a mistake the controls on the Georeferencing tab in the Review section can be used to select, move, or delete control points.
- When you are done and feel your image is well aligned with the base map, click Save to save the changes to the image.
- Click Save -> Save as New. This will save the georeferenced image to a GeoTIFF file that includes spatial information for the georeferenced image. You can use the default name, but giving it a more-meaningful name can be helpful when you access this in the future.
- Click Close Georeference to finish georeferencing.
Create a New Feature Class
Although georeferencing creates an image that you can map directly, it does not capture individual features on the map. If you want to use those features for additional mapping or analysis, you need to digitize the features into a feature class. While software and techniques exist to partially or fully automate this process with high-resolution aerial photographs, historic aerial images and maps like these need to be traced manually.
- On the View tab, view the Catalog Pane.
- Under Databases, right-click on the database for the project, and select New -> Feature Class.
- Give the new feature class a meaningful Name. The name can only contain letters and numbers, but you can give a more readable name in the Alias if desired.
- Specify the Feature Class Type based on
whether points, lines, or polygons
are the best choice for representing the features.
- Each feature class can have only one type of feature, although you can, of course, create multiple feature classes, such as a polygon feature class for buildings and a line feature class for roads.
- For this example, we will create a polygon feature class to represent buildings, some of which still exist in some form, like the American Museum of Natural History.
- In creating the new feature class, you need to determine
what Fields (attributes) you want to associate with each feature.
- For buildings, such attributes could include things like name, address, height (floors), owner, purpose, etc.
- In this case, all the map provides is the name, so that will be our only attribute.
- Qualitative attributes like names or addresses should be a Text type. Quantitative attributes like height or square feet should be either Long Integer if there are no decimal points or Double if there are decimal points.
- The Spatial Reference should usually be WGS 1984 unless you are working in a professional environment where you need something more specific like a state plane coordinate system.
- Drag the new feature class on to the map to add it to the map.
Digitize the Features
Once the feature class has been created, you can start tracing the features.
- If the image you are digitizing is pixellated (blocky) and hard to read, you can make it a little more readble by selecting the layer in the Contents pane and under Appearance changing the Resampling Type to Bilinear.
- Select the new feature class in the Contents pane and on the Edit tab, click Features -> Create.
- The Create Features pane will pop up and you can select
different types of drawing tools.
- The Polygon tool is the most common with irregular areas. Click to add vertex points and then double-click on the last point to stop.
- The rectangle tool is easiest with rectangles.
- If you need to better align your polygon virtices, Select the feature, right-click and select Edit Vertices.
- To modify the attributes of a feature, Select the feature, and right-click to edit Attributes, change the attributes as needed, and click Apply.
- When you are done tracking, on the Edit tab, click Manage Edits -> Save.
Publish the Map
All of the steps above create a georeferenced raster image and a feature class that are kept in the project geodatabase. If you are only planning on using these materials within one project, you can create maps using them directly from the project geodatabase. If you want to want to share these materials with colleagues or use them in web maps, you need to publish them to a server, in this case ArcGIS Online.
Print Map of Features
- Change base map to imagery.
- Change feature color if desired.
- Increase the transparency so the current terrain is visible under the historic locations.
- Turn on labels and adjust the Labeling Properties so the labels stand out over the map (white text with a black halo)
- Create a new print layout for the map.
- Export the map to a PDF.
The digitized vector features can be shared by publishing to a feature service.
- Set the metadata so users know what the data is and where it came from.
- Right click on the layer in Contents and select Share and Create Web Layer.
- The name should come in from the metadata.
- Share with your organization.
- After the layer is published, you should be able to create a map with the layer in ArcGIS Online.
The georeferenced map image is still a raster, so it cannot be published as a feature service. However, it can be published as a tile layer, which breaks the images into 256 x 256 map tiles that the browser or mobile map app stitches together into a complete image for the viewer.
- Right-click, select Properties and edit the Metadata to provide source information to users.
- Right-click on the image in the Contents pane and
select Sharing -> Share as Web Layer.
- Give the layer a meaningful name.
- Layer Type is Tile.
- Share with your organization.
- When the tool completes, you should now be able to create a map with the layer in ArcGIS Online.
- If the layer doesn't appear when you create the map, check the Settings tab and Show current tile details. ArcGIS Online may need a few minutes to build your tiles depending on how large your map is.
Save Project Package
You should save your project to ArcGIS Online in case you need to recover or edit any of the contents in the future.