Determining map scale values for background images.
USGS topo map images like those at www.libremap.org
are great as background images for
deed mapping. The USGS scanned their paper
maps to make those images. The paper maps have a scale of 2000 ft. per inch, and they
were scanned at 250 dots per inch. Those are
the two numbers you need to enter into DeedMapper's View>>Options>>Images
tab in order to have the program properly scale the image.
But recent posts in this newsgroup involved a Loudoun Co., VA, map image showing original
land grants in that county. How do you
determine the scale factors for such an image? Well, I recently did this, and here are
1. Open the map image in Microsoft Paint. (Right click on the image file and select
'Open With', then choose Paint.) Zoom in and
out of the image by holding the Ctrl key while using the mouse scroll wheel. Use the
window's scroll bars to move east-west and
north-south. Find a portion of the image that has an east-west or north-south road. Pick
easy to identify crossroads to that road.
Zoom and scroll to that portion of the image. The farther apart the two points are on the
screen, the more accurate the result will
2. Start Google Earth. Enter the location, in this case Loudoun Co., Va. Find the same
east-west or north-south road and the
crossroads you had previously identified in Paint. Press the ruler icon in the toolbar.
Click on one crossroad, then move the
mouse and click on the other crossroad. The ruler will tell you the distance, in feet,
between the two points you clicked. Write
3. Back in Paint, move the mouse to the same two crossroads. At each point, note the x,y
location as displayed in the bottom status
bar of the Paint window. It will say, for example, 2520, 1824 px, where px means pixel.
Write the two locations down on a piece of
4. If the road is either east-west or north-south you can just subtract the x or y values
respectively to figure the number of
pixels between the two points. For example, if the road is east-west you could have
2520, 1824 px
3114, 1824 px
The x values are different and the y values are the same or very close to each other.
You'd subtract 2520 from 3114 to get 594 pixels between the two points.
(You can use the Windows calculator. I use it all the time. Press the Start button and
enter Calc in the box.)
5. In the View>>Options>>Images tab of DeedMapper you enter the number of feet
as measured by Google Earth and the number of pixels
as calculated above.
If you can't find an east-west or north-south road, or two easily identified points
that are east-west or north-south of each other,
the arithmetic gets a bit messy. In step four above you calculate the number of pixels as
a. Subtract the first two values from each other to get the number of x pixels.
b. Subtract the second two values from each other to get the number of y pixels.
c. Calculate (x times x) plus (y times y). Take the square root. (The Calc tool has a
button for this.) The result is the number
of pixels you need.
Also, you may have noticed that we didn't calculate any "per feet" or
"per inch" values here, but we don't actually need to.
DeedMapper divides the two numbers you enter to get the feet/pixel that it really uses.
If you prefer to use "per" values, take the
Google Earth value and divide it by the number of pixels, then multiply by 100. Enter
that as the feet per inch and then use a
value of 100 dots per inch.
The values I got for the big Loudoun map are 3244 ft/inch and 100 dpi.