I am a beginner in GIS and Learning PostGIS. I came across this two terms Geom and Geog, Could anyone explain in brief. What is the best to use with map data and Do all functions support both Geom and geog.
Answer
You can find your answer in "PostGIS in Action", from Regina O. Obe and Leo S. Hsu, Edited by Manning
The difference from geometry and geographyc type:
"PostGIS 1.5 introduced a new spatial type called geography, which uses geodetic measurement instead of Cartesian measurement. Coordinate points in the geography type are always represented in WGS 84 lon lat degrees (SRID 4326), but measurement functions and relationships ST_Distance, ST_DWithin, ST_Length, and ST_Area always return answers in meters or assume inputs in meters."
What is best to use? It depends:
"When choosing between the geometry and geography type for data storage, you should consider what you’ll be using it for. If all you do are simple measurements and relationship checks on your data, and your data covers a fairly large area, then most likely you’ll be better off storing your data using the new geography type. Although the new geography data type can cover the globe, the geometry type is far from obsolete. The geometry type has a much richer set of functions than geography, relationship checks are generally faster, and it has wider support currently across desktop and web-mapping tools."
Also, take a look in an already answered question here.
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