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?
Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.
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).
