The SRID for San Francisco with meters as the unit is 26910. Let's verify this on the spatial reference website (click on an image to enlarge it).
1. Go to the website, and click on the EPSG link.
2. Type the code you're looking for (26910) in the Search Reference box and click the Search button.
3. Click on the resulting link.
4. Click on Well Known Text as HTML.
5. On the new site, you can check basic information about your SRID. Be careful not to confuse the two entries that contain the word UNIT: one of them is given inside GEOGCS (aka Geographic Coordinate System) and is not of much interest to us, while the other entry with the word UNIT outside GEOGCS is much more important and describes the unit returned by PostGIS functions. Note that you can also check the projection type here.
Alright! Take a look at the template. We've prepared something special.
As you can see, we've used a different SRID for each use of the ST_Transform function. The results are roughly the same but not identical. You can check each of these SRIDs on the websites we mentioned. Note that it is always worth checking information on more than one site, as the database of a given website may not have the desired SRID.
If you look up the SRIDs, you will see that each has a different precision and area for which it may be used. This accounts for the slight differences we see when we run the query.
