A warm up to georeferencing scanned paper maps
Evan Thornberry | GIS Librarian
two parts to this session:
- Crash course slideshow
- Hands-on activity (the rest of the time)
Session is geared toward beginners
Please feel free to ask questions
Georeferece
providing geographic context to an object
Geocode
assigning coordinates to locations (very often addresses)
Georectify
transforming an image into a geospatial data source (i.e. a scanned paper map into a GIS layer)
we’re georeferencing a map by georectifying it's image
pixels ⇢ coordinates
Why is this important?
turn historic info into "data"
visualize / analyze change over time
understand "place" in new contexts
How is this done?
step 1: find image to be georeferenced + corresponding dataset
step 2: match locations
step 3: transform image into geospatial raster data
What is needed?
a quality scanned map or aerial image
a dataset
a GIS
Ground Control Points
points on both image and dataset
locating them is a manual process
generally something that has not moved
ideally 6-10 per image
Geometric Transformations
GCP pixels coordinates ⇢ lat/long coordinates
several different types
transformations may "warp" the image
Residual Errors
difference between where the point ended up compared to the actual location of the point
used to determine consistencey between GCPs
total residual error is recorded (RMSE) when transformed
Let's open ArcMap and try this out!
follow along with this worksheet:
bit.ly/georef-handout
Use your hand out to follow along