Week Six Lab--Georeferencing

Part I of this week's lab taught georeferencing. Two UWF (north and south) raster projections were added to the map. Once added, they appeared on "Null Island" off the west coast of Africa until the layers were georeferenced and "knew" their actual location. To move the image from Null Island, I clicked on the "Fit to Display" button. Once the image was located on top of the current layer, I added control points to align the image with the actual map. A few lessons: Spread out the control points, consider the effects of shadows on building geometry, zoom into the layers for accurate control point placement, and place the control point on the unreferenced layer first.

Review the overall Root Mean Square Error to determine how close the points were to the actual location. Remove control points (view the control point table) with high RMSEs to reduce overall RMSE. Then, determine the transformation to use. The higher the transformation level, the more control points (6 for 2d Order Polynomial; 10 for a 3d Order Polynomial) are needed and the more the raster bends.

Another very interesting portion of the lab demonstrated the techniques to place an unreferenced copy of a drawn map. Once I added the survey map (as imagery), I then georeferenced the map to the referenced imagery as before. This was much more difficult because the map was much older, had limited information, and skewed excessively if control points were placed too close (next to each other versus opposite sides of the image).

Heritage Hall Georeferenced

The above image is the result of georeferencing a parcel survey and then placing it on a UWF Campus image. The control points are shown in red (parcel survey) and green (image) and the overall RMSE was 6.770932. The most efficient way to place control points was to use the edges of parking lots and the points of medians, since no buildings were drawn on the parcel survey. Again, place control points on opposite ends of the image (much like you tighten lug nuts on a tire) to reduce distortion.