# Geographic coordinates, latitude and longitude, how to determine geographic coordinates from a topographic map.

There are many different coordinate systems. All of them serve to determine the position of points on the earth’s surface. These include mainly geographical coordinates, flat rectangular and polar coordinates. In general, coordinates are called angular and linear quantities that determine the position of points on a surface or in space.

## Geographic coordinates, latitude and longitude, how to determine geographic coordinates on a topographic map.

Geographical coordinates are angular values ​​- latitude and longitude, which determine the position of a point on the globe. Geographic latitude is the angle formed by the equatorial plane and a vertical line at a given point on the earth’s surface. This value of the angle shows how far one point or another on the globe north or south of the equator.

If the point is located in the Northern Hemisphere, then its geographical latitude will be called northern, and if in the Southern Hemisphere – southern latitude. The latitude of the points located at the equator is zero degrees, and at the poles (North and South) – 90 degrees.

Geographic longitude is also an angle, but formed by the plane of the meridian, taken as the initial (zero), and the plane of the meridian passing through this point. For the sake of uniformity, the definition of the benefits agreed to consider the meridian passing through the astronomical observatory in Greenwich (near London) as the initial meridian and call it Greenwich.

All points located from it to the east will have east longitude (180 degrees to the meridian), and west longitude to the west of the initial one. The figure below shows how to determine the position of point A on the earth’s surface, if its geographical coordinates (latitude and longitude) are known. Note that the difference in the longitudes of two points on the Earth shows not only their relative position with respect to the zero meridian, but also the difference in time at these points at the same moment. The fact is that every 15 degrees (the 24th part of the circle) in longitude is equal to one hour of time. Based on this, the geographic longitude can determine the time difference at these two points.

## for example.

Moscow has a longitude of 37 ° 37&# 8242; (eastern), and Khabarovsk –135 ° 05&# 8242 ;, that is, lies east of 97 ° 28&# 8242 ;. What time do these cities have at the same moment? Simple calculations show that if in Moscow 13 hours, then in Khabarovsk 19 hours 30 minutes.

## How to determine geographic coordinates, latitude and longitude, from a topographic map.

The figure below shows the layout of the sheet frame of any topographic map. As you can see from the figure, in the corners of this map the longitude of the meridians and the latitude of the parallels forming the frame of the sheet of this map are signed. On all sides, the frame has scales broken into minutes. Both for latitude and longitude. Moreover, each minute is divided by points into 6 equal sections, which correspond to 10 seconds of longitude or latitude..

Thus, in order to determine the latitude of a point M on the map, it is necessary to draw a line through this point parallel to the lower or upper frame of the map and read the corresponding degrees, minutes, seconds on the right or left on the latitude scale. In our example, point M has a latitude of 45 ° 31’30.

Similarly, drawing a vertical line through point M parallel to the lateral (closest to the given point) meridian of the border of this map sheet, we read the longitude (east) equal to 43 ° 31’18.

## Drawing a point on a topographic map according to specified geographical coordinates.

Drawing points on the map according to the given geographical coordinates is carried out in the reverse order. First, the indicated geographical coordinates are found on the scales, and then parallel and perpendicular lines are drawn through them. The intersection of their map will show the point with the given geographical coordinates.

Based on the book “Map and Compass My Friends”.
Klimenko A.I..