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I'm new to GIS, in PostGIS, and I'm struggling with the SRID concept.
How do you choose a SRID for a database column? This is making me crazy. Why do I need to choose a SRID? What's its meaning?
Picking a SRID is it the same for geometry and geography? Setting a SRID for a geometry field seems senseless, because it should be just on a 2D plane.
When I find lat/lon on the internet or geocoding services, which is the SRID?
Doesn't a lat/lon coordinate make sense without a SRID?
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An SRID is a coordinate system. We're taught in (traditional / Arc) "GIS" to always store your data in a projected coordinate system, because we're going to eventually use some calculation like 'area', so we'd better store our data in a coordinate system that gives us that measurement.
However, PostGIS throws that concept out the window.
Here's a good Q&A started by @tmcw: Why are Data Projections relevant?
Basically, PostGIS opens up the ability to store your data in a single coordinate system such as WGS84 (SRID 4326), and when you need something like Area, Distance, or Length, you use a function to create that column from your data in a projected coordinate system that will give you a local interpretation of your data in units that you want.
So for example, I could store students and schools in PostGIS both in WGS84/SRID:4326. When I want to calculate the distance between students and the schools they attend, I call a distance function on my geometry column, but also wrap a ST_Transform function around the geometry column first to 'project' the data into State Plane CO Central (SRID: 2877). This gives me a column for the distance of each student to their closest school in feet because SRID:2877 is a projected coordinate system that stores data in Feet.
So my recommendation for you would be to store your data in a Geometry (data type) column in SRID 4326 (as oppose to a geography data type which does not support projections / transformations).
