The earth is not flat, Columbus!
[mailto:firstname.lastname@example.org]On Behalf Of Dave Bird
Sent: Saturday, February 21, 2009 1:42 AM
Subject: [DMU] Background Map Scaling - My Experience
I have been using USGS maps downloaded from
;. These are 7.5' maps and download
as tifs. The map information, where ever I found it, was that the scale
was 1:24000 which comes out to 2000 ft/in. Loading the maps into
Photoshop Elements and looking at the image size, I got the dpi which
was 72 pixels per inch. I think this number is irrefutable. Unless a dpi
is not the same as a pixel/inch?
The first thing was that DM would not display these maps as tifs. Nor
would they display as gifs, which were made with the save as function in
PSE. jpgs worked OK, which I generated the same way, so that is what I
have been using.
But there was doubt about the scale. So, I looked up some surveyors
terms on the web. A pole = 16.5 ft, 1 acre = 160 square poles, 1 square
mile = 640 acres, and 1 mile = 320 poles.
The USGS maps as downloaded have no scale information anywhere in them.
I happened to have a USGS map (hardcopy) which I bought some years ago
of my region of interest which happens to be in the Mammoth Cave area of
Kentucky. Using the scale on the hardcopy, there were a couple of
features which were exactly 1 mile apart. Then in DM, I measured between
those two features (using the + cursor/button) and found it to be way off.
Then I made a one square mile plat in DM each side of which has 320
poles (one mile), and moved it over the map features from the hardcopy.
Adjusting the ft/in scale in the View Options Images tab to get the
right fit, it ended up being 483 ft/in!
Now, the scale seems to better match the deed location descriptions. My
only question is why such an odd scaling number? Seems like a downloaded
USGS map would not be such a challenge to fit.
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