Question
How can I resolve the Mixed SRID Geometries error message in MapThat?
Answer
The Mixed SRID Geometries error message displays in MapThat when the user ticks to display a Data Layer that has incorrect or missing SRID’s. By running a SQL query within your underlying spatial database you can determine the current SRID of your spatial data.
For example within PostGIS use the following SQL query:
SELECT Find_SRID('public', 'lsoa_select', 'geom');
The results show that the spatial data has no SRID, which could have been an error when importing the data originally.
To fix this issue simply run a SQL query that updates the SRID of the spatial data.
For example within PostGIS use the following SQL query:
SELECT UpdateGeometrySRID('lsoa_select','geom',27700);
The results show that the spatial data has now had its SRID updated to 27700. 
The corresponding Data Layer will now display in MapThat.