Since Thailand is located near the Equator, the projection that should be used is a cylindrical projection (Mercator). This method will grant the line that is parallel to the equator and meridian to intersect each other at a right angle. On one hand, the map direction and shape are correct especially the area near the point of contact. On the other hand, the further from the point of contact, the more error of direction and shape.
From that, knowing the exact scale factor is one of the important components in geographical surveying and measurement especially when doing transformation from Cartesian coordinate to Spheroid coordinate accurately. Furthermore, it can eliminate the error and distance differences between measuring from the real world and the distance on the coordinated map.
There are 3 kinds of Scale Factor.

1. Grid Scale Factor (GSF) is used for adjusting the distance measured in the real world with the distance on the map projection coordinate such as UTM (Universal Transverse Mercator). As we know the plane map is unavailable to display the distance of the earth's curvature directly, so we use GSF to calculate the correct distance.
2. The elevation Scale Factor (ESF) is used to adjust the elevation height from the center of the earth. The accurate ESF should be calculated using an average of the actual radial distances from the center of the ellipsoid to the endpoints of the line (Earth surfaces).
3. Combined Scale Factor (CSF) is the combination of GSF and ESF. CSF is used to calculate the high accuracy distance for example: building structures, pipeline installation, railway profiling, etc.

Have you ever wondered why sometimes when measuring distances with a Total Station, the distance is shorter or further than the design even though the Total Station is just calibrated from the service center? The further the distance is, the bigger the difference.
This is because our world is spheroid. If we want it to map to display flatly, we must use the “Map Projection”.
Those calculations can be done easily through many professional geospatial software such as Trimble Business Center (TBC). The software can calculate and adjust the GSF and ESF related to the imported data which helps users know the true distance and coordinate promptly.