[GIS] Converting from kilometers to degrees distances

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We have geometries in a Postgis database with SRID WGS84 and we have found lookups directly in degrees to be much faster than in kilometres, because the database can skip the projections I would think.

So, having a reference point in coordinates and a distance threshold in kilometres, is there a somewhat precise way to convert the distance to degrees for the lookup?

Example: Finding all locations within 10 Km of POINT(-3, 40). How do I convert the 10 Km to a degree value?

Best Answer

The length of degree in north-south is about the same so you could use 1/110574 degree/meter as a factor. However, the farther to south or north you go the bigger the error is in east-west direction.

For example, take these two shapes which have a 1 degree buffer in EPSG:4326 transformed into EPSG:32630 (UTM zone 30N). First one is from 40°N and the second from higher north at 70°N.

SELECT ST_Transform(ST_Buffer(ST_GeomFromText('POINT (-3 40)',4326),1),32630);
SELECT ST_Transform(ST_Buffer(ST_GeomFromText('POINT (-3 70)',4326),1),32630);

Draw them side by side and it is easy to see that in UTM projection the area of the degree buffer is oval and it gets quite narrow at 70°N. The height of both ovals is about 220 km and widths at 40° and 70° are about 170 km and 80 km, respectively. I wouldn't call even the oval at 40°N as somewhat precise. For more accurate results you can use this calculator http://www.csgnetwork.com/degreelenllavcalc.html

Related Solutions

ArcGIS Desktop – Calculate Distance to Nearest Points in Lat/Long Using ArcGIS or R

Although this isn't an ArcGIS solution, your problem can be solved in R by exporting your points from Arc and using the spDists function from the sp package. The function finds the distances between a reference point(s) and a matrix of points, in kilometers if you set longlat=T.

Here's a quick and dirty example:

library(sp)
## Sim up two sets of 100 points, we'll call them set a and set b:
a <- SpatialPoints(coords = data.frame(x = rnorm(100, -87.5), y = rnorm(100, 30)), proj4string=CRS("+proj=longlat +datum=WGS84"))
b <- SpatialPoints(coords = data.frame(x = rnorm(100, -88.5), y = rnorm(100, 30.5)), proj4string=CRS("+proj=longlat +datum=WGS84"))

## Find the distance from each point in a to each point in b, store
##    the results in a matrix.
results <- spDists(a, b, longlat=T)

PostGIS – Calculate LineString Length from WGS84 After Transforming from OSGB 36

The length functions work differently with 3D linestring geometries:

ST_Length - returns 2D distances for geometry types, and oddly 3D distances for geography types (but not in this question)
ST_Length_Spheroid - returns 3D distances for geometry types

Your example is in 3D, so it will calculate the 3D length with ST_Length_Spheroid and the 2D length with ST_Length on a geometry.

However, if you always want 2D lengths, you can force the geometry to a 2D linestring using either ST_Force2D or ST_Force_2D function (the name change in PostGIS 2.1).

SELECT ST_Length(geom) AS length_27700,
  ST_Length_Spheroid(ST_Transform(ST_Force2d(geom), 4326), spheroid) AS length_spheroid_2d,
  ST_Length_Spheroid(ST_Transform(geom, 4326), spheroid) AS length_spheroid_3d
FROM (
  SELECT 'SRID=27700;MULTILINESTRING((
      423216.279 574665.249 0,
      423206.315 574708.077 44.649,
      423158.458 574896.911 242.525,
      423132.406 574993.061 344))'::geometry AS geom,
    'SPHEROID["WGS 84",6378137,298.257223563]'::spheroid
) AS f;

  length_27700   | length_spheroid_2d | length_spheroid_3d
-----------------+--------------------+--------------------
 338.39264086563 |   338.515774984236 |   482.622196178051

Best Answer

Related Solutions

ArcGIS Desktop – Calculate Distance to Nearest Points in Lat/Long Using ArcGIS or R

PostGIS – Calculate LineString Length from WGS84 After Transforming from OSGB 36

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