# How to find the nearest point projected on the road network?

I would like to know if there is a function at pgRouting that given a point (lon/lat) can give you the projection of that point to the road network .

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With "projection", do you mean "find the spatial reference" or "find the nearest point projected on the road network"? – Mike T Apr 15 '12 at 21:40
Thanks for your reply Mike .I think the second one ! but what do you mean by spatial reference? – john Apr 16 '12 at 17:48
spatial reference systems are things like UTM vs latitude/longitude vs Google Mercator, etc. Some folks call these "projections" – Mike T Apr 16 '12 at 22:38

There are linear referencing functions in PostGIS to help you out project the location of a point along a line. For example, if you have a road, and a point of interest (POI) that is along the side of the road, you can:

Here is the SQL to extract the interpolated POI on the road:

``````WITH data AS (
SELECT 'LINESTRING (50 40, 40 60, 50 90, 30 140)'::geometry AS road,
'POINT (60 110)'::geometry AS poi)

SELECT ST_AsText(

FROM data;
``````

Returns `POINT(44.4827586206897 103.793103448276)`, which is close to the POI, but projected on the road.

The tricky things you might run into is to locate the closest road LINESTRING and/or, your road network might be a MULTILINESTRING, which needs to be broken down into LINESTRINGs in order to work with the above. Also, linear referencing systems work best with projected (i.e., non-Lat/Lon) data, particularly if your data are far north/south from the equator.

Update An easier function that can use other geometries, including MULTILINESTRINGs is ST_ClosestPoint. For example, the closest POI on a road network is found using:

``````WITH data AS (
SELECT 'MULTILINESTRING ((20 30, 40 70, 40 110),
(40 110, 70 160, 80 190),
(40 110, 25 118, 10 140))'::geometry AS road_network,
'POINT (40 130)'::geometry AS poi)