#4840 closed defect (fixed)
ST_Azimuth with geography provides wrong results
| Reported by: | postlude | Owned by: | pramsey |
|---|---|---|---|
| Priority: | medium | Milestone: | PostGIS 3.1.2 |
| Component: | postgis | Version: | 3.1.x |
| Keywords: | Cc: |
Description
Hi, I noticed that the azimuth angles reported by ST_Azimuth are obviously wrong in some cases.
The following simple example demonstrates this. The central point C is located at 5°E,55°N. All other points are shifted by 1° in the respective directions:
SELECT
degrees(ST_azimuth(C,N)) AS az_n,
degrees(ST_azimuth(C,NE)) AS az_ne,
degrees(ST_azimuth(C,E)) AS az_e,
degrees(ST_azimuth(C,SE)) AS az_se,
degrees(ST_azimuth(C,S)) AS az_s,
degrees(ST_azimuth(C,SW)) AS az_sw,
degrees(ST_azimuth(C,W)) AS az_w,
degrees(ST_azimuth(C,NW)) AS az_nw
FROM (SELECT
ST_GeogFromText('SRID=4326;POINT(5 55)') AS C,
ST_GeogFromText('SRID=4326;POINT(5 56)') AS N,
ST_GeogFromText('SRID=4326;POINT(6 56)') AS NE,
ST_GeogFromText('SRID=4326;POINT(6 55)') AS E,
ST_GeogFromText('SRID=4326;POINT(6 54)') AS SE,
ST_GeogFromText('SRID=4326;POINT(5 54)') AS S,
ST_GeogFromText('SRID=4326;POINT(4 54)') AS SW,
ST_GeogFromText('SRID=4326;POINT(4 55)') AS W,
ST_GeogFromText('SRID=4326;POINT(4 56)') AS NW ) points
The result is (I manually removed some decimals for easier readability)
az_n | az_ne | az_e | az_se | az_s | az_sw | az_w | az_nw ------+-------+-------+--------+------+--------+--------+------- 180 | 29.16 | 89.59 | 149.39 | 360 | 329.39 | 269.59 | 209.16
This is not what I expect. I am well aware that a difference of 1° in N and E does not result in an azimuth of 45° due to the convergence of the meridians. The azimuth angles az_ne, az_e, az_se and az_w seem correct (I checked these angles in a conformal map projection in QGIS). However, what I expect is
az_n | az_ne | az_e | az_se | az_s | az_sw | az_w | az_nw ------+-------+-------+--------+------+--------+--------+------- 0 ! | 29.16 | 89.59 | 149.39 | 180 !| 211 ! | 269.59 | 331 !
What was going wrong for az_n, az_s, az_sw and az_nw?
The version of my DB is
version | postgis_version ---------------------------------------------------------------------------------------------------------------------------------+--------------------------------------- PostgreSQL 13.1 (Ubuntu 13.1-1.pgdg18.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0, 64-bit | 3.1 USE_GEOS=1 USE_PROJ=1 USE_STATS=1
installed via the apt.postgresql.org repositories for Ubuntu 18.04.
Something has changed in 3.1 and up, I get results as expected in 3.0 (modulo negative angles)
Come to think of it, the wrong results in 3.1+ may well be a result of a bad fix trying address those negative angles. (Mine are even worse that yours?)