M easurementM odel ofDigitalM aps Based on Geographic Coordinates

To construct a digitalmap based on geographic coordinates is a bettermethod to bring digitalmaps in different coordinate systems or projection zones into one system or joint them. In the measurementof digitalmaps based on geographic coordinates, the deviation between the reference ellipsoid surface and the earth s surface results in the discrepance between the calculating value on the reference ellipsoid surface and the real surveying value on the earth s surface. Because a projection plane ismore close to the earth s surface, the calculating value on the projection plane ismore close the real surveying value on the earth s surface. Therefore, themeasurementmodelofdigitalmapsmay be setup by establishing a compensation coordinate system. In thismode,l the compensation projection plane is the surface ofaverage elevation ofcalculated objects, and the centralmeridian ofprojection is the average longitude line of calculated objects, so the calculating result is most close to the real surveying value. This model (method) is more suitable for the regions with a small area and a requirementofhighmeasuring accuracy.