[Qgis-user] How to create a QGIS grid but not the usual way

[John Antkowiak]

Hi. I'm new to QGIS. I've been up all night reviewing the Training Manual and watching tutorial videos looking for someone to address my situation. I'm overtired and I'm sure I'm missing something obvious.
I'm trying to create a large-format paper wall map to show all the roads in my county, and I have to create an atlas to do it.
For sake of this question, let's assume the county is approximately 19 miles east-west and approximately 28 miles north-south. The key word here is approximately - and I suspect this task requires precision that I don't know how to find. 
All the lessons for creating grids from which the atlas will be generated instruct me to do it the same way: specify the desired horizontal and vertical intervals. That's not what I want.
What I want is to divide the extent by three, whatever interval that happens to be. And I can't figure out precisely the total dimension, in miles, of the map extent. In case this is relevant: the data layers are projected in NAD83 / North Carolina (ftUS) but the QGIS GUI at the bottom right says "Unknown CRS." 
I created a layer for this exercise - don't ask me to explain how I did it - whose only feature is a rectangle covering the area to be divided. This layer I call "Extent."
I try to use the Measure tool to get the exact east-west distance of the polygon. BUT. I have to manually select the start and end points, and I have no confidence that the points I'm selecting precisely match the limits of the polygon because when I measure the north and south horizontal lines, I get two different values. It's a rectangle. The values ought to be the same.
Am I going bout this all wrong? How do I create this 3 x 3 grid?
Thanks - 
John A.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/qgis-user/attachments/20210515/0731c969/attachment.html>