A digital image is recorded during the geo referencing. It can be an aerial photo, a scanned geological map, or a topographic map image and can add geographic information to the image so that the GIS or the map software can “place” the image in the real real-world image. The digital image process is completed by selecting pixels and assigning them geographic coordinates. In rare cases, you can already know the geographic coordinates of certain pixels in an image. Often a non-georeferenced image is georeferenced onto an existing image in which geographic information such as DRG, DLG or DEM is already embedded (see data resources).
Special software is generally required to complete the geocoding process. Eratost Technologies offers features for this technique as well as numerous remote sensing software packages. For GeoPads purposes, the digital images of geological maps available in the National Geological Map Database are the most pressing need for georeferencing.
Techniques to refer to the data:
The georeferencing of geodata can be done in two ways. First of all, you can use 3D coordinates where the center of the earth serves as the origin of the three axes. It is a popular method for scientific applications, but it is not considered possible to apply it to points on the surface of the earth.
The second method is known as the projection of the points. With this method, 3D coordinates are expressed as a plane with a certain height above them. This technique allows you to flatten the curved area of a small area on a flat surface for reference purposes. The coordinates of the plane point to a certain point within the projection. It is measured in distances, north, and east of the origin.
The Height Measurement:
To understand the use of the geographic reference point for data mapping by the projection method, it is important to understand the height measurement. Usually, it can be measured the heights from sea level. However, this method lacks precision because the sea level is constantly changing. Increase the complexity and cost of references.
There is another metric called ellipsoid with approximate values that adjust to sea level. Local reference systems differ for each local mapping agency. However, the calculation of multinational data can be difficult due to cross-border convergences.