Merge pull request #162 from specklesystems/Kate/QGIS_CRS_offsets

Update qgis.md

user/qgis.md

@@ -108,6 +108,26 @@ Steps to receive the data:
 ![Receiving data](./img-qgis/QGIS_05_receiving.gif)
 
 
+### Custom project center
+Modifying project center is useful when you need to: 
+ - Send data (so you can receive correctly in a non-GIS application)
+ - Receive data (if data was sent from non-GIS application).
+
+You have several options to choose in order to set the custom project center. 
+
+#### Adding offsets
+You can choose to use the existing Coordinate Reference System (Project CRS) and modify it's Speckle properties by adding X- and Y- offsets. This option will only be affecting Speckle properties on Send/Receive and will not change anything for you while you are working on a QGIS project. Use this option when your project requires a use of a specific CRS. 
+
+For example, if you use a projected CRS (e.g. EPSG:32631) without the offsets and send the Eiffel Tower outline to Speckle, when receiving in Rhino geometry will be located thousands kilometers away from the origin. But if before sending you set the offsets Lat(y) as 5411939.08 and Lon(x) as 448253.52, you will receive the Eiffel Tower right in the middle of your Rhino canvas, while still preserving the required CRS (e.g. EPSG:32631). 
+
+Lat(y) and Lon(x) offsets should be specified in the units of the current Project CRS, and in the correct order. Note, that when you copy the coordinates from QGIS canvas, the Lat/Lon order might be different depending on the CRS used (Lat is usually marked with N-North, and Lon is marked with E-East, both values can be positive or negative). 
+
+#### Creating custom CRS
+Create a custom Coordinate Reference System, based on Traverse Mercator if you don't have to use a specific CRS and want to have minimal size and shape distortions. Specify the origin Lat, Lon in geographic coordinates to create a custom CRS with the required origin and change your Project CRS. 
+
+#### Adding angle to the True North
+You can also add an angle to the True North if your non-GIS application require so. It will only affect Speckle object properties during Send/Receive operations and will not be visible in QGIS project.
+
 ### Transformations
 From 2.14 version onwards there is a new functionality helping to send a full 3d context maps for the further use in CAD, BIM or any 3D software. This can be applied by clicking "Apply transformations on Send" before sending the data. 
 ![transforms_intro](./img-qgis/transforms_intro.png)