[postgis-users] How do you obtaining a WGS-84 (4326) coordinate from a coordinate that has no SRID

[Rick]

On Wed, Aug 19, 2009 at 5:53 PM, Eric B. Powell <ebpowell.chip at gmail.com>wrote:

> ILAN BENISTY wrote:
>
>> Hi,
>>
>> I am dealing with a problem where I am importing an AutoCAD description of
>> an airport (DXF) into PostGIS.
>> I have a Python script which will import each coordinate to a PostGIS
>> geometry with SRID = -1 (since I do not have real world coordinates, but
>> Cartesian coordinates from a point of origin).
>> I do, however, have a lat/long reference point, which represents the
>> origin of my x/y CAD coordinates.
>> I would like to be able to store my CAD coordinates points as Long/Lat
>> (SRID=4326).
>> Any suggestions?
>>
>> Thanks,
>>
>> Ilan
>> ------------------------------------------------------------------------
>>
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>>
>>
>
I'll answer for Ilan, we work together

>
>>  What country is the site in? What units are used for the drawings grid?
> Do you know the geographic datum?


Country: Canada, Units: meters, Datum: WGS84

I expect the solution is straightforward, but it is not obvious to us.


> It may require a fair amount of pre-processign before the data can be
> accurately (de)projected to LatLong.
>
> Once the details of the drawing grid are known, there is a python wrapper
> for Proj4 which could be used to (de)project the points on the way into the
> database.


We are very experienced programmers, but have a limited knowledge of SQL.
We've been working with it for a few years, but at this point have been able
to get by quite nicely with the basics.

We have a fair knowledge of Geodesy.  It was my major, 20 years ago.

--
I'll give you the use case:

Currently we have a geometric database using PostgreSQL geometry types to
represent aerodromes.  The origin of the grid is the centroid of the
aerodrome, and that we have a Geodetic position for.  We use it, amoung
other things, to plot radar targets.  The radar data arrives on the same
grid, so up to this point our data registers, we display the live radar data
as SVG in any browser that is not IE with one second updates.  This works
well.

However, for many reasons, we have to go to a geodetic system.  We'll still
use the same metric grid with an origin at the centroid for radar data, but
to integrate with other systems we must maintain a geodetic database.

The aerodrome data arrives as DXF.  We have written an import filter in
Python that parses in the DXF and converts it to Postgresql geometry.  It
also maps the layer names to a standard.  Ilan, the author of this thread,
is tasked with converting this to PostGIS so that we may merge with other
systems that maintain geometry in WGS84.

The general coverage of the data is three or four kilometers.  Surface radar
data arrives at a resolution of one meter, so if our accuracy is within 10cm
we are more than good.  We assume a reasonably accurate centroid position
and a Cartesian plane that does not extend more than 3 kilometers in any
direction from the origin, so curvature error is not an issue.

--
The problem:

We have a metric cartesian data set.

We have a geodetic coordinate for the centroid of the aerodrome, which is
the origin of our metric Cartesian data.

We need to convert this to geodetic to exchange data with other components
of the air navigation system.

--
It seems to me that the solution might be to have PostGIS give us a
coordinate for the centroid, transform the DXF coordinates during the parse
operation, and then import it into PostGIS.

We don't know how to properly do that, and it's quite possible that that is
not the ideal solution.

As I said, we are new to PostGIS, but we are keen to play.



>
>
> Eric
>
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
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>



-- 
Cheers!
Rick
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