Using your Hardcopy Map Grids with Degrees-Decimal- Minutes

Due to the converging parallels (lines of longitude) at the poles, the distance between lines of longitude varies at different latitudes. The table below displays the distance in feet of 1/1000^th (.001) of a minute of longitude at various latitudes.

Latitude Longitude

(Degrees) (1/1000 Minute)

24 5.56 ft

25 5.52 ft

26 5.47 ft

27 5.43 ft

28 5.38 ft

29 5.33 ft

30 5.28 ft

31 5.22 ft

32 5.17 ft

33 5.11 ft

34 5.05 ft

35 4.99 ft

36 4.93 ft

37 4.87 ft

38 4.80 ft

39 4.74 ft

40 4.67 ft

41 4.60 ft

42 4.53 ft

43 4.46 ft

44 4.39 ft

45 4.31 ft

46 4.24 ft

47 4.16 ft

48 4.08 ft

Most maps have a GPS grid that is 200 feet high and 160 feet wide. Some may differ but the plotting process is the same as long as you know the distance between the grid in feet. First I will show you the difference in distance of longitude at different parallels. Lets say we have a reading of 95 45.000, the last 3 number represent 1/1000^th of a minute. The distance in feet of 1/1000th of a minute of longitude varies in distance. At 38 degrees the length of 1/1000^th of a minute in feet is equal to 4.8 feet. The latitude varies slightly and is approximately 6 feet in distance at 1/1000^th of a minute of latitude. Latitude on your GPS will increase as you go north and decreases as you go south. Since we are west of the Prime Meridian longitude units are negative. Longitude decreases as you go west (increasing in the negative value) and increases as you go to the east (decreasing in negative value) This is important to remember.

Let’s say we are going to plot a position between the grid to go north. If we know our present position we just increase the last number of the latitude by 1 and we have moved

6 feet north (i.e. 38 95.001).You can increase to go north and decrease the number to go south. Let’s say you want to go north 100 feet and the grid is 200 feet high you just divide 6 into 200 and it will give 33.33. Just add the 33 to the last numbers on your GPS and it will move you approximately 100 feet north. If you multiply 33.33 x 6 it will equal 199.98 feet. This is about as close as you can plot. Now we will plot the longitude distance. Let’s say you want to go 80 feet to the west after you go north. 1/1000^th of a minute at the 38 parallel is approximately 4.08 feet. If the grid is 160 wide and we want to plot the map to go west 80 feet. You would divide 4.08 into 80 feet and this will give you 19.6 thousands of a minute. Just add 19.6 or 20 too your longitude readings and you will go west approximately 80 feet. If you have any doubt just get on the lake or walk with your GPS and you will see how the numbers change going north or south and east or west.