Lab 2: Georeferencing

Goals:

The goal of this lab is to understand how to georeference data that has no spatial reference information. Then new data can be created and used to solve spatial analysis problems.


Methods:

The first step of the lab was to add data that has spatial information attached, and is very accurate. In this case, it is a shapefile of transportation networks and roads of Eau Claire county. Then a basemap is added, and finally the non-georeferenced image was added. This image is shown below in Figure 1 and is from a 1878 historic map of Eau Claire.

Figure 1. A historic map of Eau Claire that has no spatial information attached. It will be georeferenced to make the map more accurate to modern day.

Then, using the georeferencing toolbar, control points were added. These control points connect a point on the historic map to a point on the shapefile for the transportation network, georeferencing points on the historic map. In this case, 15 control points were used. Control points where placed on features like street intersections or distinctive areas. Water features were not used as they can change between years. The idea is to have enough control points and have a low RMS error, meaning the true point and ungeoreferenced point are close enough. The map is shown in Figure 2 with control points added.

Figure 2. Note the red crosses with numbers, these are the control points. This image shows an already transformed raster.

A spline transformation was used to adjust the raster. Spline was chosen because it is a true rubbersheeting method and is focused more on local accuracy compared to global accuracy which works well with the small raster area.

Part two of the lab features creating data, specifically water features as polygons. A new feature class was created in a file geodatabase. The boundaries of Half Moon Lake, Chippewa River, and the Eau Claire River from the 1878 map were then digitized. An example of the digitized features is shown below in Figure 3.

Figure 3. Digitized polygons. These are the water features from the 1878 map overlayed on the current features. 

Then, to compare, the same water features were digitzed using the 2018 World Topographic Map boundaries.

Results:

Figure 4. The final georeferenced raster. Obviously, some features are not 100% spatially defined to what the map says, but the map is more spatially accurate. RMS error is under 1 as the spline transformation is used.  

Figure 5. Comparison of water features between the 1878 map, and the 2018 map. Obviously the main difference is that the features from 2018 are more spatially accurate, but they are also more detailed. The water features have sharply defined edges and seem more true to life, whereas the ones from 1878 seem more generalized.

Sources:

Eau Claire and Medford[Photograph found in David Rumsey Map Collection]. Retrieved from https://www.davidrumsey.com/luna/servlet/detail/RUMSEY~8~1~4181~480085# (Originally photographed 1878)

Eau Claire County. [Master_Centerlines feature class (clipped)]. Raw data.

ESRI. (2018). World Topographic Map [Map].