I imported an OSM file for the state of Delaware into the PostGIS Database via connecting to it through QGIS. I would like to build a query to retrieve the nearest roads to the (latitude,longitude) points I have.

When I imported to QGIS, it created 4 layers: one for linestring, one for points, one for multilinestrings, a and once for multi polygons. I believe the only ones I need would be lines. So far I have imported my table of 1000 ( latitude, longitude) points. I need to write a query that for each point, will return the highway tag of the nearest road to that point. I've written a query that casts each latitude longitude into a point geometry:

select ST_AsText(ST_MakePoint(end_latitude::float, end_longitude::float)
from gis.delaware_trip_slices

Each row output here would be for example Point(37.2,75.32) etc. Now, I just have to find a way to return the highway tag for the nearest road to each of these points. Do you have any pointers how I would go about doing this.

I saw a query like this online, but I don't know what this means:

Select *
From towns 
order by the_geom <->
Limit 5

My guess is somehow I could make the above query a temporary table. Then I would for each value in the temporary table search for the nearest road. Does anyone have any pointers

  • 2
    You'll find all relevant information about the concept needed and its implementation in e.g. this answer and its links, and/or via the tag knn on GIS.SE
    – geozelot
    Commented Oct 2, 2023 at 18:42
  • 1) see the operator doc, 2) coordinates should be in the longitude;latitude format, 3) beware that <-> uses the Cartesian distance, which is meaningless with geometries in 4236
    – JGH
    Commented Oct 2, 2023 at 19:44

1 Answer 1


<-> operator tells the distance between two geometries. Sorting tables using this operator is very fast if there is a spatial index created on the tables.

The following query selects one point at a time and then finds the nearest road for it. You didn't specify the schema of your tables so you have to fix all the column names etc.

  gis.delaware_trip_slices points
    SELECT osm_lines.name, osm_lines.geom <-> ST_Transform(ST_SetSRID(ST_MakePoint(end_latitude::float, end_longitude::float), 4326)), 3857) AS dist
    FROM osm_lines
    WHERE osm_lines.tag = 'highway'
    ORDER BY dist
    LIMIT 1
  ) roads
  • I used <-> in the ORDER BY statement is that ok? Commented Oct 4, 2023 at 20:09
  • The last part of the query is like: CROSS JOIN LATERAL ( SELECT hlines.id, osm_lines.line_geometry, osm_lines.highway, osm_lines.geom_text FROM osm_lines ORDER BY osm_lines.line_geometry <-> hpoints.point_geometry LIMIT 1 Commented Oct 4, 2023 at 20:10

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