PREPARING YOUR USGS 7.5 MINUTE MAP FOR USE WITH COORDINATE FINDING INFORMATION
PREPARING YOUR USGS 7.5 MINUTE MAP FOR USE
WITH COORDINATE FINDING INFORMATION
Before you can begin to find a coordinate from a location or a location from a coordinate, you must first
draw your coordinate lines on your map. The tick marks that need to be connected are marked with the degrees, minutes and seconds and they are black in color. These tick marks are located, on the North to South sides, 7 9/16 inches from the top corners of the map and 7 9/16 inches from the bottom corners of the map. On the East to West ends of the map (top and bottom), these tick marks are located 6 1/16 inches from the West corners of the map and 6 1/16 inches from the East corners of the map. On your map, you will find four marks like this +, only bigger. Before you actually draw your coordinate lines on the map, check to make sure your straight edge is going across these marks. If it isn't, you are in the wrong place. If you are in the correct place on the map, when you draw your four coordinate lines, all four of the lines will go through the + marks. These + marks, mark the intersection of all your coordinate lines. After you get all four of your coordinate lines drawn, you will have nine equal sections on your map and you will be ready to start figuring your coordinates and, or, locations.
FINDING REAL DISTANCE FROM A MEASURED DISTANCE
ON YOUR USGS 7.5 MINUTE SERIES MAP
2.640 inches measured on your map = One mile in real distance.
To find real distance from distance measured on map (in inches), take the measured distance and divide 2.640 into it. This will give you the distance in miles. Then you multiply this figure, in miles, by 5280 (the number of feet in a mile) and that will give you the distance in feet. EXAMPLE: Measured distance 1.028 inches-divided by 2.640 = .389 of a mile, then multiply .389 x 5280 = 2054 feet (rounded off to the nearest foot).
TABLE FOR CONVERTING FRACTIONS TO THOUSANDTHS OF AN INCH
(Rounded off to the nearest thousandth)
TOOLS NEEDED FOR FIGURING COORDINATES
AND, OR, LOCATIONS ON YOUR MAP
More precise measurements can be made, using a caliper, or a measuring compass, but satisfactory results can be obtained by using a small ruler that is graduated in increments of 1/16 of an inch or less. By using the table above to convert your measurements , in inches, to the closest thousandth of an inch and using a calculator, it is easy to obtain satisfactory results, even with the use of a ruler.
MAPS OTHER THAN 7.5 MINUTE SERIES
All the information for the 7.5 minute series of maps would apply to any other size map, except for the "seconds per inch" figure. This would have to be refigured. For a 15 minute series map, for instance, to get the "seconds per inch" figure, you would have to convert the 15 minutes into seconds. Since each minute contains 60 seconds, that figure would be 900 seconds, (15x60). Next, you would measure the length and width of the map. You then take the measured distance, in inches, and divide it into the number of seconds (900) and that would give you the "seconds per inch" figure.
You would need to calculate this for the East to West direction and then again for the South to North direction.
Note: Some maps, including the 15 minute series may not be exactly 15 minutes in either latitude or longitude.
In this case you would
need to check to see how many minutes or degrees are actually on the map, convert that to seconds and divide the
size, in inches, of the map into the number of seconds. Don't take for granted that any map is any given size. Always
check to make sure before figuring any coordinates. For a better explanation of map size and area covered on the
different map series,
click here.
Any time you are figuring coordinates for any serious excursion, especially on maps other than the 7.5 minute series,
you should always figure some test coordinates for a known area to make sure your measurements and figures are
giving you correct and reliable coordinates.
Instructions For Figuring Coordinates and Locations
jcarl@cinci.rr.com
