# [OSM-dev] projection for calculating distances in PostGIS

Marcus Wolschon Marcus at Wolschon.biz
Wed May 19 15:56:56 BST 2010

```On Wed, May 19, 2010 at 2:53 PM, Emilie Laffray
<emilie.laffray at gmail.com> wrote:
> The page is interesting due to the utmzone function. This will allow to know
> what projection to use if you want a good approximation. It return a SRID
> that you can use to do a ST_Transform.

Ah, with only the line "Find UTM (WGS84) SRID for a point (in any SRID)"
I was not quite sure what it was supposed to do. Thanks for the clarification.

> But what you want to do is going to be very painfully slow especially if you
> are going to work with geometries other than points, since it means that you
> have to reproject the data for each geometries you are going to work with
> potentially.

Why should that be slow?
I´m calculating a projected bounding-box for each element once ahead of time
and storing these. When doing the queries I only need to project my
query-location
to find the few elements where my query-loction is within the bounding-box.
Then I transform the few geometries found to compare the distance and sort.

The following page will work in any units that you want.
> http://www.bostongis.com/PrinterFriendly.aspx?content_name=postgis_nearest_neighbor_generic

Only if the reference-point and the bounding-boxes have the same projection
and the distance()-function will use that projection to calculate the distance.
So this will be highly distorted for WGS84 as 1 meter is not the same distance
in lat as it is in lon.
So basically back to square one. Finding a good projection.

> One other possibility
> instead of using a nearest neighbour functionality is to do a search with
> ST_DWithin applying a ratio depending on the latitude.

That could work.
Calculate a WGS4-bounding box with a width and height depending on the loction
of the reference-point and select all geometries who intersect it.
Could I use utmzone() to calculate a good projection, then project my
reference-point
to it, create a bounding-box with that SRID of +- N meters, project that back to
WGS84 and do the range-query?
Then use that SRID in the distance-calculation of the resulting elements that
where found to intersect the bounding-box?

Marcus

```