Georeferencing is the process of taking a scanned map or image and assigning it a real-world location. When an image is georeferenced it is assigned a spatial coordinate system and can be used with other geographic data in ArcGIS and other popular GIS platforms. In this tutorial we will download a Sanborn Fire Insurance plan of Calumet, Michigan from the Library of Congress, bring it into ArcGIS Pro software, and georeference it. This tutorial assumes some familiarity with GIS and ArcGIS software.
In this section we will download a high-resolution TIFF image from the Library of Congress. If you already have another image that you are working with, please skip to the next section.
Step 1: Go to the Library of Congress Website and on the top search bar select Maps from the dropdown menu and search for Sanborn Maps Laurium. From the Results list select Sanborn Fire Insurance Map from Laurium, Houghton County, Michigan from 1917-09.
Step 2: Click to view the Images in Sequence.
Step 3: Find and select Image number 23 from the list of sheets. On the bottom of the image where it says Download, select TIFF from the dropdown menu and hit Go. The TIFF file will download to your computer.
In this section we will georeference the Sanborn Map. When georeferencing it is always a good practice to identify features in which you will georeference the historic map to. One of the easiest and most common features for georeferencing an item to are street intersections, which generally do not change much over time. One problem is typically with historical maps, the older they are the more difficult they can be to georeference. Also, sometimes an area has gone through so much change that it will no longer contain any of the features it once did and is now unrecognizable. For this reason, in certain cases it may not be possible to georeference a historic map.
Step 1: Open ArcGIS Pro and start a new project. Select Map and Name the project Laurium_1917_Sanborn.
Step 2: Zoom the map to Laurium. On the Map tab hit the Add Data Button and select Data. Navigate to the folder where you downloaded the Sanborn Map to and select it. ArcGIS Pro will probably calculate statistics and warn you the image does not have a known coordinate system. That is okay, we are going to assign it one now. In this tutorial we will be georeferencing in the default coordinate system of WGS84 Web Mercator but you can georeference an image in other coordinate systems as well. For example, for older maps or aerial photography you may want to use NAD83 UTM Zone 16N. In this tutorial you don't need to do anything with the coordinate system because we are using the default of WGS84 Web Mercator.
Step 3: With the layer highlighted under Drawing Order go to the Imagery Tab and click the Georeference button. Now click the Fit to Display button and the Sanborn map image should appear on your screen.
Step 4: Now lets look at the street intersections on the Sanborn Map. We want to georeference them to the street intersections on the basemap. Click on the Add Control Points button under the Georeference Tab. Cross hairs will appear, click on the intersection of 4th ST. and KEARSARGE ST. on the Sanborn Map. Now toggle the Sanborn Map layer off, and find the intersection of 4th St. and Kearsarge St. on the basemap and click on it. Toggle the Sanborn Map layer back on and you should see the Sanborn Map shift into that location but it still doesn't look quite right. We need to add more control points to give ArcGIS a better idea of where the Sanborn Map is actually located on the Earth!
Step 5: Next, we are going to add a control point for the intersection of 3RD ST. and PEWABIC, which is this case is in the upper-right hand corner of the Sanborn Map. Again, hit the Add Control Points Button and place a control point at the intersection of 3RD St. and Pewabic on the Sanborn Map. Toggle the layer off, and find the intersection on the basemap and click the intersection as we did in the previous step.
Step 6: Now turn the layer back on. Now you can see the Sanborn Map is in a far more precise location than it was previously. Lets add at least a couple more control points to improve the georefencing accuracy. This map only covers a small area, so it will not require many control points. Maps that cover a larger area will more than likely require a greater amount of control points.
Step 7: I have added control points at the intersections of 5th ST and Pewabic, 3rd ST and Hecla St, and 4TH ST and Hecla St. Now the Sanborn Map is laid in a very precise location over the basemap that I am pleased with. You can adjust the transparency of the Sanborn Map overlay at any point during the georeferencing process to see how it compares with the basemap by going to the Raster Layer tab and adjusting the Transparency percentage.
Note: If at any point you make a mistake and need to delete a previous control point, you can open the Control Point Table. Select the desired control point, and press the Delete button.
Step 8: When you are happy with your georeferenced map, go to the Georeference Tab and click Save and then click Close Georeference. Your map is now georeferenced and ready to be used in GIS software.