August 3, 2026
The QGIS program - Topo maps, part 3 - cropping
The map sources most useful to me are scans of historic USGS map sheets.
These scans are available in several forms, what I find most useful is
the PDF. These scans include map margins, legends, and such.
It is desirable to crop off that information, especially if two
adjacent sheets are intended to be joined together.
QGIS can crop a raster using a polygon, so the necessary thing is to
generate a polygon (a rectangle defined by long/lat limits).
Some suggestions for doing this are:
- the Create Grid tool
- the QuickWKT plugin
- the Shape digitizing toolbar
To create a grid, use Vector -- Research Tools -- Create Grid.
Set grid type to Rectangle (Polygon).
Enter long,long,lat,lat
Leave the spacing values set to 1.0
I enter -110,-109,32,33
It works! I get a rectangle. It seems to use EPSG:4326 by default. Nice and easy.
Feedback? Questions?
Drop me a line!
Tom's Geology Info / tom@mmto.org