When creating a new raster, how to determine a raster's upperleftx, upperlefty?

Question

I'd like to create a raster from point data, averaging the point data over a given area and placing the results into a given pixel/cell. With each pixel/cell as 50m x 50m.

I'm new to this and have a beginner question.

I see that in PostGIS 2.0 there is a ST_MakeEmptyRaster() function, but what are the units for width, height, and pixelsize. And how do you determine the upperleftx, upperlefty points? Is this just random, or is there a convention used to determine this point?

raster ST_MakeEmptyRaster(integer width, integer height, float8 upperleftx, float8 upperlefty, float8 pixelsize);

Answer 1

The units are not random but depend on your SRID. If you have no SRID then they are "unknown". The coordinates are wherever you want them to be in the real world and in the units of your SRID (be that decimal degrees or meters or whatever). In other words, the location and size of the raster is up to you.

You could work this out by getting the extent of your point data (possibly adding a margin around the edge to reduce edge effects). Alternatively, if you have a raster that covers the area in question, you could pass that into ST_MakeEmptyRaster as a 'seed' instead of all the coordinates. Or, a "cheap-and-dirty" way to get your coordinates if you don't have another raster handy and don't know how to get the extent of your point data (or can't be bothered to calculate your buffer) is to digitize a polygon in an appropriate SRID (or simply buffer your point data). Then convert to a raster either setting all the data to Null or using this as a seed for ST_MakeEmptyRaster.