On Earth, the normal reference datum is sea level. On other planets, such as the Moon or Mars, the datum is the average radius of the planet. Each of the different types of projections have strengths and weaknesses, and knowledge of the advantages and disadvantages of a particular map projection will often help you to choose what map you want to use for a particular project. Topographic maps Much of the information discussed above is applicable to all types of maps. Much of what follows is information that can be found on many general maps, but for the rest of this exercise the emphasis will be on what information is contained on a topographic map, and where it can be found.
All maps have a horizontal scale.
Topographic maps also have a vertical scale to allow the determination of a point in three dimensional space. The degree of variation changes, depending on where you are. As a result, the direction your compass needle points will not always be the same as north on a map. Once you have determined the bearing you want to go on the map, you need to understand how that translates to actually getting from point A to point B in the field using your compass. You can use your map and compass to triangulate between three points and locate yourself pretty accurately.