Using geodb for geography calcualtions (e,g distance between lat/long coordinates)

[Kit Kline]

Hi all,

I am struggling to use h2 geodb to perform geographic calculations...like the below link for postgris.

I would be grateful for any tips or pointers from anyone who has achieved this (simply put I want to select out the distance in miles (or any unit!) between two Points holding lat/long co-ordinates)

http://postgis.refractions.net/docs/ST_Distance.html

Any help much appreciated

Cheers

Kit

[Justin Deoliveira]

Hey Kit,

Actually the version linked from the wiki on github doesn't have the distance function (lame i know) but it has since been added. If you try this build out it should work as you would expect.

  http://files.opengeo.org/geodb/geodb-0.7-app.zip

I have also updated the wiki to include a link to the newer 0.7 build. Let me know how that works.

-Justin

Kit

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Justin Deoliveira

OpenGeo - http://opengeo.org

Enterprise support for open source geospatial.

[Kit Kline]

H - thanks for such a prompt response - I have 0,7 with ST_Distance in it OK (and can select distance between two Point)- but that seems to be doing distance between two Cartesian points on flat surface and not distance between two lat long on curved earht surface - which makes sense. How do I get ST_Distance to process the points as lat/lon on a curved surface? Or....I'd like to either use an ST function that does the great circle calculation or something like this postgris example below.

Any advice or a code snippet much appreciated - ps loving  this library - great work! 

-- Geography example -- same but note units in meters - use sphere for slightly faster less accurate
SELECT ST_Distance(gg1, gg2) As spheroid_dist, ST_Distance(gg1, gg2, false) As sphere_dist 
FROM (SELECT
	ST_GeographyFromText('SRID=4326;POINT(-72.1235 42.3521)') As gg1,
	ST_GeographyFromText('SRID=4326;LINESTRING(-72.1260 42.45, -72.123 42.1546)') As gg2
	) As foo  ;

  spheroid_dist   |   sphere_dist
------------------+------------------
 123.802076746848 | 123.475736916397

[Kit Kline]

Just follow up to my own past in case helps.

I'm sure I have just committed a terrible hack (knowing little about GIS) - but I only need this function for my unit tests - the production MYSQL database provides the spatial functions needed.

I added the code below to one of my own classes (but I guess it would go in GeoDB more formally?). It includes a haversine implementation (but maybe that should have come out of JTS or Hatbox - did not have time to research that :)

Anyway - it works but I'm pretty sure it's architecturally ugly in terms of reuse etc etc !!!

/**

* Provides an implementation of ST_Distance_Sphere using the Haversine formula

* @param wkb1 the first location

* @param wkb2 the second location

* @return the distance between the two locations

*/

public static double ST_Distance_Sphere( byte[] wkb1, byte[] wkb2 ) {

if ( wkb1 == null || wkb2 == null ) {

return -1;

}

Geometry g1 = GeoDB.gFromWKB(wkb1);

Geometry g2 = GeoDB.gFromWKB(wkb2);

return getHaverSineDistance(g1, g2);

}

/**

* Calculate the Haversine distance between two lat/long geometries

* @param wkb1 the first location

* @param wkb2 the second location

* @return the distance between the two locations

*/

private static double getHaverSineDistance(Geometry g1, Geometry g2) 

{

double lat1 = Math.toRadians(g1.getCoordinate().x);

double lng1 = Math.toRadians(g1.getCoordinate().y);

double lat2 = Math.toRadians(g2.getCoordinate().x);

double lng2 = Math.toRadians(g2.getCoordinate().y);

double dlon = lng2 - lng1;

double dlat = lat2 - lat1;

double a = Math.pow((Math.sin(dlat/2)),2) + Math.cos(lat1) * Math.cos(lat2) * Math.pow(Math.sin(dlon/2),2);

double c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));

return EARTH_RADIUS_IN_MILES * c;

}   

 And added the following to my schema: 

CREATE ALIAS ST_Distance_Sphere FOR "uk.co.thirdbridge.core.tools.GeoTools.ST_Distance_Sphere";

[Justin Deoliveira]

Hey Kit,

Right, the jts distance and really all of the operations will do the math assuming a cartesian space.

It would be great to have a distance function that did the match on a spheroid. Basically just poaching from what postgis does.

  http://postgis.net/docs/manual-2.0/ST_Distance_Spheroid.html

  http://postgis.net/docs/manual-2.0/ST_Distance_Sphere.html

It would be even better if we could have these functions in JTS. I'll ping Martin Davis about this to see if it's anywhere on the JTS roadmap. 

Regardless of that though I think your approach is perfect fine. I think it's one of the most powerful aspects of h2 / geodb, the ability to very easily implement your own functions. Nice work!

-Justin

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[Nagy Attila]

Hello Kit,

your code fragment helps me very much. Thank you for that.

I just observed that the latitude/longitude coordinates are switched (x=longitude, y=latitude).

        final double lng1 = Math.toRadians(g1.getCoordinate().x);

        final double lat1 = Math.toRadians(g1.getCoordinate().y);

        final double lng2 = Math.toRadians(g2.getCoordinate().x);

        final double lat2 = Math.toRadians(g2.getCoordinate().y);

Regards,

Attila