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Lesson 2. Determine the exact 4-digit coordinate of the grid square containing a specified point on a topographical map. Determine the 6-digit grid coordinate of a point on a topographical map to within meters. Given a 6-digit grid coordinate, determine the identity of the feature located at that coordinate.

Many Marines have difficulty with understanding the grid coordinate system. But it is not like that at all. Because of the number of Marines who have difficulty with grid coordinates, we will teach you grid coordinates using two different analogies.

This analogy is covered in paragraph In paragraphs and we will explain grid coordinates a second time using non-algebraic methods. The map has lines running north, south, east and west.

These lines which are called grid lines form small squares, 1, meters on each side, called grid squares. The lines that form grid squares are numbered along the outside edge of your map.

The precision of a point location is shown by the number of digits in the coordinates; the more digits, the more precise the location. A 4-digit grid coordinate, such as , identifies an area on the map that is 1, meters x 1, meters. A 6-digit grid coordinate, such as , identifies an area that is meters x meters. How big an area do you think an 8-digit grid coordinate identifies?

Note that grid lines are identified by a set of digits. Algebra Analogy a. Plotting numbers on a quadrant. In algebra, you are often required to plot numbers, such as 3,2 and 10,11 , in a quadrant fig Fig In case you have forgotten, the first number in the point that you are trying to plot is the "x" number. In our second example -- 10,11 -- the number plotted on the "x axis" is " The second number, in this case "11," is plotted on the "y axis.

If you did this properly, then guess what? This is one of the cardinal rules in map reading. Algebra and 4-digit grid coordinates.

The only major differences in locating grid coordinates on a map and plotting points on a quadrant in algebra is that in algebra, the points are surrounded by parentheses and the "x" and "y" numbers are separated by a comma! There are a few minor differences. On a map, you only deal with the "upper right hand quadrant. Single digit numbers, such as 4 and 9 have a "0" placed in front of them as a "place holder" thus being expressed as "04" and "09".

Finally, grid lines are drawn on a map to help you locate your points. Remember, this will identify an area that is 1, meters x 1, meters a grid square. You now know how to plot a 4-digit grid coordinate. Finding grid square Algebra and 6-digit grid coordinates. Suppose that you needed to get a little more precise when plotting a point on a quadrant.

Instead of plotting 30,50 , suppose you had to plot 30 3,50 7. How would you do it? To write the algebraic point When you do that, it is written Remember, a 6-digit grid coordinate identifies an area that is meters x meters. Finding a 6-digit grid coordinate.

Algebra and 8-digit grid coordinates. An 8-digit grid coordinate is done the exact same way. For grid coordinate , the first four numbers, , apply to the "x axis. The second set of four numbers, , apply to the "y axis. An 8-digit grid coordinate identifies an area that is 10 meters x 10 meters. Now recite the magical incantation, "ekac fo eceip" backwards ten times to release yourself from the stagnant morass of incomprehension. In paragraphs and we will teach grid coordinates again, without using the algebra analogy.

Even if you were "blessed" with taking algebra in school, read paragraphs and anyway to get a better understanding of grid coordinates.

First, identify the grid square in which it is located. This is grid line Write "11" down on a piece of paper. Now look at the numbers on the side of the map and read UP until you identify the last east-west grid line before arriving at hill Write "43" down right beside the " Note that the point where these two grid lines intersect is in the lower left hand corner of the grid square.

In other words, grid squares are identified by the grid lines that intersect in their lower left hand corner. You have, therefore, located a point to the nearest 1, meters by using a 4-digit grid coordinate fig Determining a 4-digit grid coordinate. Imagine dividing the grid square into smaller squares. The coordinates of a point in such a grid square will have six digits numbers. Each of the grid squares in figure is 1, meters long and 1, meters high.

One of the grid squares is divided into smaller squares--each meters long and meters high. In a 6-digit grid coordinate, such as , the first 3 numbers are the "read right" part of "read right, then up;" whereas, the last 3 numbers are the "then up" part. Write the vertical coordinate for point as Write the horizontal coordinate of point C as Do not actually draw the lines within the grid square fig , since such lines would obstruct other information.

You can determine a 6-digit grid coordinate by approximation, or by using your coordinate scale on a protractor. Use of the coordinate scale is explained in the next paragraph. Determining a 6-digit grid coordinate. Use of the Coordinate Scale If you require a more accurate 6-digit grid coordinate than can be obtained by estimation, you should use a coordinate scale fig Coordinate scales on a protractor.

There are three coordinate scales located on your protractor: ,, ,, and , Use the one that corresponds with the scale of the map you are using. In most cases this will be , To locate point C in grid square using the coordinate scale on your protractor, use the following procedures: a. Coordinate scale. Place coordinate scale on map. Keeping the horizontal line of the scale directly on top of the east-west grid line in this case, grid line 50 , slide it to the right until the vertical line of the scale touches the point point C for which the coordinates are desired fig Examine the two sides of the coordinate scale to ensure that the horizontal line of the scale is aligned with the east-west grid line, and the vertical line of the scale is parallel with the north- south grid line.

Add to this value the number which tells how far into the grid square point C is. In this case, it is meters fig Next, determine your UP reading by first reading the value of the horizontal grid line below point C Add to this value the number which tells how far point C is up in the grid square. In this case it is meters. You now have the complete UP reading of fig When determining both your right and up reading, round your value to the closest number on your coordinate scale.

Locating Points on a Map You have learned how to determine the grid coordinates of a given point on a snap to within meters 6-digits. Next you need to know the method of locating a point on your map when given a 6-digit grid coordinate. To locate a point on the map, using the coordinate scale on your protractor, use the following procedures: a. Locate the correct arid square. To locate the correct grid square, determine the 4-digit grid coordinates from the given 6-digit grid coordinates.

It is the first two numbers of your UP reading. Plot the 6-digit coordinates. The procedures for plotting your grid coordinates are listed as follows: 1 Place the proper coordinate scale of your protractor with the zero-zero point at the lower left hand corner of the grid square , keeping the horizontal line of the scale directly on top of the east-west grid line.

Place the coordinate scale. Slide coordinate scale to the right. In this case the UP value of the vertical scale is at the meter reading 2 fig





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