I'm using QGIS and I would like to measure the distance between the centroid of King County in the US and the centroids of all other US counties. I have imported the shapefile and I have executed the "centroid" tool. However, I am not able to set "King County" as a starting point. When I do "Distance Matrix", there is no way to set King County as the starting point. I would like to have "King County" as the reference point and then measure the distance from it to all other counties.
The best way to calculate this is SQL Spatial, which you can access on a shapefile from the QGIS DB Manager SQL Panel, which treats your shapefile as a 'virtual layer' in which you can run SQLite spatial functions.
For my example, I want to find the distance from the centroid of Denver county to the centroid of all other counties in CO:
Here is the SQL:
select d.name as "from " , c.name as "to" , ST_Distance(ST_Centroid(d.geometry), ST_Centroid(c.geometry)) as "dist_dd" from co_counties as c , co_counties as d where 1=1 and d.name = 'Denver' and c.name != 'Denver'
The SQL basically sets up the same layer twice: c, and d using commas to select both tables, rather than joining them.
I can then alias both tables, then set which features from which table I want to use:
- c is the state without denver
- d is just denver
Then use the ST_Distance function on both the geometries, while converting both to centroid points using ST_Centroid.
Note: my data is in WGS84, so the distance is fairly meaningless.
However, if your data is in a USA-level projected coordinate system, this will work.
PostGIS would be the ideal system if you had to do this more often.