2. Map projection must be used to establish the mathematical foundation when drawing a map, but there is deformation in each projection. On the topographic map larger than 1 ∶ 1 10,000, the projection deformation is very small, which can be represented or measured by the same scale-main scale.
On the map of a vast area with a smaller scale, different parts have obvious deformation, so it can't be represented and measured on the same scale. In order to eliminate the influence of projection deformation on map survey, a composite scale is drawn for use according to projection deformation and map main scale.
The compound scale consists of the main scale scale line and several local scale scale lines, which are divided into longitude scale and latitude scale. According to the meridian length ratio, the compound ruler made by calculating the corresponding field length of the basic ruler segment is called meridian ruler, which is used to measure the length of the line segment along the meridian or near the meridian direction; According to the ratio of weft length, the composite ruler made by calculating the field length corresponding to the basic ruler segment is called weft ruler, which is used to measure the length of a line segment along the weft or approximate weft direction.
5 a segment equivalent to the length of the standard line, and then marked with the main scale; When measuring the length of a line segment in other places, it is necessary to determine which scale to use according to the longitude or latitude of the line segment.