Reading Coordinates
Grid lines are a series of straight lines intersecting at right angles forming squares on a topographic map. Horizontal grid lines are numbered west to east. Vertical lines are numbered south to north. The basic rule about reading grid coordinates is to read right on the vertical grid lines, then up on the horizontal grid lines, or “Read Right then Up”.
The map has vertical lines (top to bottom) and horizontal lines (left to right). These lines form small squares 1,000 meters on each side called grid squares. The lines that form grid squares are numbered along the outside edge of the map picture. No two grid squares have the same number.
A grid square’s coordinates are found by combining the identities of the horizontal and vertical grid lines that intersect at the lower left hand corner of the grid.
The most accurate way to determine the coordinates of a point on a map is to use a coordinate scale. You do not have to use imaginary lines; you can find the exact coordinates using a Coordinate Scale and Protractor.
First, locate the grid square in which the point is located. The number of the vertical grid line on the left (west) side of the grid square is the first and second digits of the coordinates.
The number of the horizontal grid line on the bottom (south) side of the grid square is the fourth and fifth digits of the coordinates.
To determine the third and sixth digits of the coordinates, place the coordinate scale on the bottom horizontal grid line of the grid square containing Point A, with the vertical scale going through the point. Line up the vertical line so it goes through your point, leaving the horizontal line along the horizontal grid line.
On the bottom scale, the 100 meter mark nearest the vertical grid line provides the third digit, 5. On the vertical scale, the 100 meter mark nearest Point A provides the sixth digit, 3. Therefore the six-digit grid coordinate is 115813.
Grid squares are 1000 meters square.
They are subdivided into 100 meter and 10 meter marks.
The Grid Coordinate Scale helps you measure the grid square and determine a more accurate grid coordinate.
A 4-digit grid coordinate is accurate to 1000 meters
A 6-digit grid coordinate is accurate to 100 meters An 8-digit grid coordinate is accurate to 10 meters
To find the grid coordinate of a point:
► Determine the scale of your map. Find the correct scale to use on your protractor.
► Determine the grid square the point is in. This gives 4 digits: 12xx56xx.
► Next, using the correct scale on your protractor, align the bottom right corner of the protractor scale to the bottom right corner of the grid square.
► Slide the protractor to the left until the line forming the right edge of the scale intersects the point.
► Read off the horizontal and vertical position: 49551885
Finally, a grid coordinate isn’t complete without the Grid Zone Designator. This is a 2-letter designation that is unique to a 100,000 meter area. (This is where the grid numbers start over again at 00.) The Grid Reference Box in the Marginal Data shows the Grid Zones that are on that map. The correct depiction of a grid coordinate ALWAYS includes the Grid Zone. i.e. ET 99450076, which is 100,000 meters from FT 99450076.
Measuring Distance
You can use your map to measure the distance between two places. The maps are drawn to scale. This means that a certain distance on a map equals a certain distance on the earth. The scale is printed at the bottom and top of each map (i.e. Scale 1:50,000). This means that 1 inch on the map equals 50,000 inches on the ground. Or 1 centimeter on the map equals 50,000 centimeters (or 500 meters) on the ground. To change map distance to miles, meters, or yards, use the bar scales at the bottom of the map. Take a ruler or the edge of a paper and mark on it the straight line distance between your two points. Then, put the ruler or the paper just under one of the bar scales and read the ground distance, in miles, meters, or yards. The bar scale here shows a ground distance of 1500 meters. To measure distance along a road that isn’t straight, break up the road into little bits that are more or less straight, measure each leg, and add them together. You can do this on your straight piece of paper by making a tick mark where the road turns, then rotate the paper and make the next tick mark at the next turn. In the end, you have a series of tick marks, but one complete line on your paper.
Direction
Direction – a course along which someone or something moves – is usually expressed in map reading as a degree or azimuth, or as a variant of North, South, East, and West. From any point on a plane, there are 360 degrees leading out from it in a circle. In almost all coordinate systems, north is portrayed at 0 degrees.
True North is a line from any point on the earth’s surface to the North Pole. All lines of longitude are true north lines. True North is usually symbolized in marginal information with a star.
Magnetic North, as shown by the compass needle, points to the north magnetic pole, which is not the same as the geographic North Pole. It is shown in marginal information as a half-arrow.
Grid North is the north that mapmakers put on a map, dependent of the map projection used. It is shown in the marginal information by the letters GN on a vertical line.