Previously, you filtered the SRS list for WMS. Are you wondering what you will find inside this demo? Let's see.
Open the SRS list demo application. Wow, there are 4,956 items in the list! Yes, you just filtered items for WMS; but all supported SRSs are still there.
In the Search textbox, type in the project code for the basic projection, 4326; then press Enter.
Click on the projection code to show the projection detail. Along with the Well Known Text description of the SRS, there is also a map showing you the area of validity. For 4326, it is the planet's surface:
Repeat these steps to review 3857, which is the Google Mercator projection.
This gives you an idea of how each projection (4326 and 3857, in this example) is defined. Each projection is defined by several parameters formatted in the WKT format.
If you have custom projections, they'll be included in this list. You can also check your data_dir/user_projections folder for...