Re: [geomesa-users] Transformations, Edge effects?

[Jim Hughes <jnh5y@xxxxxxxx>]

Hi Marcel,

The functions you are calling are actually GeoTools methods.  To see a 
list available, you can check out your the WFS GetCapabilities document 
from GeoServer (1) under the ogc:Function_Names tag.

Distance is a tricky thing:  When ones ask for a distance calculation, 
the units will be determined by the Coordinate Reference System (CRS).  
GeoMesa makes the assumption that all your data is longitude / latitude 
which is EPSG:4326.  In that CRS, the unit of measurement is degrees.

In order to get a 'more helpful' answer, libraries like GeoTools 
GeodeticCalculator or our GeoMesa wrapper (2) can take two points 
specified in lon-lat and return the distance in meters.  Those libraries 
use the Haversine formula or the Vincenty's formula (3).

For the Point(0,0), it is on a corner of a GeoHash.  In our 
implementation, GeoHashes contain their bottom and left edge, so this 
point is in the 's' 5-bit GeoHash.

One easy way to see duplicate data in GeoMesa is if you have entered the 
same data multiple times without specifying the feature id.  If that's 
not what has happened, you may have found a bug.  If you can write up 
some steps to reproduce, I'm happy to check things out.

Thanks,

Jim

1.  For example: 
http://your-server/geoserver/ows?service=wfs&version=1.0.0&request=GetCapabilities

2.  GeoMesa's Scala wrapper about the GeoTools GeodeticCalculator:
https://github.com/locationtech/geomesa/blob/master/geomesa-utils/src/main/scala/org/locationtech/geomesa/utils/geohash/GeomDistance.scala
Unit tests/examples of use:
https://github.com/locationtech/geomesa/blob/master/geomesa-utils/src/test/scala/org/locationtech/geomesa/utils/geohash/GeomDistanceTest.scala

3.
https://en.wikipedia.org/wiki/Haversine_formula
https://en.wikipedia.org/wiki/Vincenty's_formulae


On 09/04/2015 12:54 PM, Marcel wrote:
> Hey,
>
> I played around with some geomesa transformations like strConcat() and 
> distance(). This returns the distance in degrees which looks kind of 
> unfamiliar to me. Is there a transformation, which returns the 
> distance in meters or kilometers (given two points)? Which distance do 
> you calculate (euclidean distance, distance using haversin formula or 
> based on an ellipsoid)?
>
> Looking at the results of another query I noticed that one record 
> occurs 4 times (Point(0, 0)). I could imagine that there is the 
> boundary of a geohash and this point intersects with all of the four 
> geohashes around. Do I have to remove these duplicates afterwards?
>
> Thanks again,
> Marcel.
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