August 4, 2026

The QGIS program - Topo maps, part 3 - quads

The word "quads" is short for quadrangles. In my world, I only care about 2 kinds, 15 minute quads and 7.5 minute quads.

A 15 minute quad covers a 1/4 degree by 1/4 degree square. These become narrower and narrower rectangles as you move away from the equator. For maps in my area, the limits are things like: 

32.0   -110.0
32.25  -109.75
32.5   -109.5
32.75  -109.25
33.0   -109.0

A 7.5 minute quad covers a 1/8 by 1/8 degree square. So you get limits like: 

32.0     -110.0
32.125   -109.875
32.25    -109.75
32.375   -109.625
32.5     -109.5
32.625   -109.375
32.75    -109.25
32.875   -109.125
33.0     -109.0
In the case of my first project (Fagan), we have the Mount Fagan quad with the limits: 
31.875 (31:52:30) to 32.0
-110.75 (-110:45) to -110.625 (-110:37:30)
Feedback? Questions? Drop me a line!

Tom's Geology Info / tom@mmto.org