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I have a spatial table in SQL Server with point features, and connected it to QGIS which neatly shows all points.

Now I want to select points in this table (and show them in the map) within a certain radius from a certain point.

I am building a plugin in QGIS to perform this, based on a postal code api (which gives a point) and a radius.

My question is: how to select these points with Python? With the setSubsetString command? If yes, what should be the filtering criterion string, and if not, how then?

Background

I have already working code to request the postal code api, and centre the map on the resulting point (lon, lat).

The code for the postal code point is:

map_pos = QgsPointXY(lon, lat)

The code for the layer to be queried / filtered is:

pr = QgsProject.instance().mapLayersByName('Project')[0]

The points in this layer are in the geography type field 'Position'.

Then I want to use pr.setSubsetString( 'filterstring') to select the points in this layer that are within a radius of say 1 km around point map_pos.

Which 'filterstring' should I use?

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Im not sure about the measurements (coordinate system in degrees, distance in meters) but the results doesnt look unreasonable:

layer = iface.activeLayer()

#https://gis.stackexchange.com/questions/266360/pyqgis-when-we-search-based-on-the-distance-between-two-points-is-the-measurem
distance = QgsDistanceArea()
distance.setEllipsoid('WGS84')

search_distance = 50000 #m
map_pos = QgsPointXY(14.4602,57.5751)

to_select = []

for point in layer.getFeatures():
    p = point.geometry()
    p.convertToSingleType()
    if distance.measureLine(p.asPoint(), map_pos) <= search_distance:
        to_select.append(point.id())

layer.select(to_select) #If you want to select
#layer.setSubsetString('id IN{}'.format(tuple(to_select))) #Or filter

enter image description here

But there must be a MSSQL equivalent of something like postgis ST_DWithin which must be alot faster than above code?

| improve this answer | |
  • For instance SELECT * FROM Project WHERE Position.STDistance(GEOMETRY::Point(10, 30, 4326)) <= 0.5. My former post that was not answered tried to get a clue how to use such a query in Pyqgis. – SWP_IB May 7 at 12:22
  • Then maybe you can use Execute SQL and pass output ids to pyqgis select – BERA May 7 at 12:23

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