Introduction to Map Use

Interpolation between Grid Marks

1. Find the labeled grid marks on either side of the point of interest.

2. Subtract the smaller grid value from the larger. This is the distance in grid units from one mark to the next.

3. If the grid marks do not extend across the map, using a piece of paper or a ruler and a PENCIL, use the grid marks on the margin of the map to lightly draw the grid near your point of interest

4. Measure the distance on the map (in inches or cm) between the marks. We’ll call this D.

5. Measure the distance from the smaller grid mark to the point of interest. We’ll call this d.

6. Find d/D, the fraction of the distance between the grid marks you must travel to get from the grid line to the point of interest.

7. Multiply d/D by the grid units found in 2.

8. Add this to the small grid mark value.

9. Do any mathematical adjustments need to get your answer in a standard form.

Cautions!

1. the larger grid value is not always above or to the right of the smaller grid value. Make sure you measure from the smaller grid value to the point.

2. the grid lines are not always parallel to the sides of the map.

3. it is not necessary that your measurements be perpendicular to the grid, but they must be on the same line.

4. check your answer. It should be between the value of the two grid lines and be closer to the value of the line your point is closer to. If not, check your work.

EXAMPLE: find the latitude of X

24 deg __ __ 24 deg

X

22 deg __ __ 22 deg

24 deg __ ____________________ __ 24 deg

D

X

d

22 deg __ ____________________ __ 22 deg

24 deg – 22 deg = 2 deg between marks

d = 1.66 cm

D = 2.45 cm

d/D = 0.678

0.678 * 2 deg = 1.36 deg

X is at 22.00 deg + 1.36 deg = 23.36 deg