Converting 1 meter to degrees at specific location using PostGIS

Question

Given a point in EPSG:4326 I need to know how many degrees (approx) 1 meter is at this location using PostGIS. Feels like http://postgis.net/docs/manual-2.1/ST_Line_Interpolate_Point.html is useful here but I cannot figure it out.

Another way to phrase this question is: What is one point (X) here that would give me 1 meter back when running the following query?

  ST_Distance(
   'SRID=4326;POINT(Input point)'::geography,
   'SRID=4326;POINT(X)'::geography) AS geography_distance
   ;

I would happily accept a point in any direction.

Here is the same question but using "FME's Tester" Converting 1 meter to decimal degrees using FME

This is going to depend on direction. 1 metre N-S next to the pole is a very different number of degrees to 1 metre E-W (which could be 360 degrees if you are standing next to the pole!) — Spacedman, Apr 29 '22 at 08:16
I mocked up a PL/pgSQL function of pieces of code I use to avoid costly projections and rule-of-three calculations. Takes into account a given angular direction of distance and the variance in its degree representation, or defaults to longitudinal direction. Assumes Earth is a sphere. — geozelot, Apr 29 '22 at 09:21

J. Monticolo · Accepted Answer · 2022-04-29T08:32:17.433

You can execute the following query, you can modify coordinates in CTE start_coords :

WITH start_coords AS (SELECT 5 AS long, 40 AS lat),
start_point AS (SELECT *, ST_MakePoint(long, lat)::geography AS geog FROM start_coords)
SELECT
    sp.lat,
    sp.long,
    ST_Distance(
        sp.geog,
        ST_MakePoint(sp.long, sp.lat + 1)::geography
    ) AS one_deg_lat_north,  -- in meters
    1 / ST_Distance(
        sp.geog,
        ST_MakePoint(sp.long, sp.lat + 1)::geography
    ) AS one_meter_lat_north,  -- in degrees
    ST_Distance(
        sp.geog,
        ST_MakePoint(sp.long, sp.lat - 1)::geography
    ) AS one_deg_lat_south,  -- in meters
    1 / ST_Distance(
        sp.geog,
        ST_MakePoint(sp.long, sp.lat - 1)::geography
    ) AS one_meter_lat_south,  -- in degrees
    ST_Distance(
        sp.geog,
        ST_MakePoint(sp.long + 1, sp.lat)::geography
    ) AS one_deg_long_east,  -- in meters
    1 / ST_Distance(
        sp.geog,
        ST_MakePoint(sp.long + 1, sp.lat)::geography
    ) AS one_meter_long_east,  -- in degrees
    ST_Distance(
        sp.geog,
        ST_MakePoint(sp.long - 1, sp.lat)::geography
    ) AS one_deg_long_west,  -- in meters
    1 / ST_Distance(
        sp.geog,
        ST_MakePoint(sp.long - 1, sp.lat)::geography
    ) AS one_meter_long_west  -- in degrees
FROM start_point sp
;

ST_MakePoint(sp.long, sp.lat + 1)::geography <-- This is not corresponding to 1 meter right? I do not think this solution is correct — Sebastian, Apr 29 '22 at 09:09
Look at the entire code, the part you mention is to compute, from the coordinates of the original point, a new geography point with an additionnal degree for the latitude. Then it compute the distance between the original point and this new point to get the number of meters for 1 degree, so 1 degree / number of meters gives you the number of degrees for 1 meter, see the SQL comments for the unit. — J. Monticolo, Apr 29 '22 at 09:14

Converting 1 meter to degrees at specific location using PostGIS

1 Answers1