### Spatial Reference System

The Spatial Reference System interface is used to define or change the SRID of a layer, and also to set or change the map display SRID.

It is accessible through:

Layer Explorer > Right Click over a Layer-name > Inform SRS...

Map Display > Right Click inside the map display area > SRS...

The icons related with SRS are and can be added to the tool bar. The last icon, called UnknownSRID, is used to reset the Map Display SRID.

When more than one layer are selected, the first valid SRID among the selected layers is used to set the map display SRID and draw them. The Map Display SRID can also be set to a desired one and in this case this one will be used to draw the selected layer.

At the interface it is shown a small list of the most recently used SRIDs, and a complete list with all Geographic, Projected and User Defined SRS.

To choose a SRID follow the stpes below:

1. SRSs recently used - select the SRID from here or go to item 2.
2. SRSs Available - select the SRS from one of the lists (Geographic, Projected, User Defined)
3. Filter - optionally, to help finding a SRS, enter a meaningful word (or a SRID) to see a subset of SRSs which contain that word (or that SRID) at the list of SRSs available.
4. Proj4 description - it is a informative field containing all parameters of the chosen SRID.
5. Click OK to set the SRID to the layer or to the Map Display.

Hints: As an example, fill the word UTM in Filter field and see all SRID that have this word in its definition. Try to fill Filter with the following words, one at once and see the SRSs showed: Corrego Alegre, Sirgas, 2916, 29168, etc.

### Usage tips

The addition, removing and edition of SRSs using this interface are under development. However, you can edit the configuration file srs.json and update the definitions available there.

If you wish to include a user defined SRS you have to provide a not used before SRID greater than 100000, a name, and a valid PROJ.4 description for it. It will be listed under the "User defined SRS" division of the interface.