[OSGeo Africa] EPSG 205x

Tue Jul 21 10:30:54 EDT 2009

Hi Andrew

A bit of clarification on what I said earlier:

SA's LO system is a modified Transverse Mercator (Gauss Conform) in which your metres South positively increases. To the east of the Central Meridian your values decrease, while to the west they increase  and are positive

So your example outputs would be correct for a standard Transverse Mercator , but incorrect for the LO system we use

Graeme

>>> "Andrew de Klerk" <andrew at quartex.co.za> 07/21/09 4:04 PM >>> 
Hi Graeme

Thanks for the response. Yes, I am well aware of the resource available
through spatialreference. This is where my confusion comes in. if you run
the following:

cs2cs +proj=latlong +datum=WGS84 +to +proj=tmerc +lon_0=31 +datum=WGS84 

and then enter:
30.352 -29.623

You will get:
-62758.77       -3278498.65 0.00

This is of course the correct answer. Which immediately tells me that the
definitions in place can handle our projection systems, and it is merely a
factor of syntax. Please correct me if I am wrong.

Andrew

-----Original Message-----
From: africa-bounces at lists.osgeo.org [mailto:africa-bounces at lists.osgeo.org]
On Behalf Of Graeme McFerren
Sent: 21 July 2009 03:26 PM
To: 'Africa local chapter discussions'
Subject: RE: [OSGeo Africa] EPSG 205x

Hi Andrew

This question needs a bit of disambiguation (oops, I sound like
wikipedia...) and discussion

I remind you btw of the excellent resource www.spatialreference.org

The Hartebeesthoek94 coord system is EPSG:4148
(http://www.spatialreference.org/ref/epsg/4148/ ) defined in proj format as:
+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +no_defs 

Projected systems can of course be based on this, but if you are
specifically after the LO flavour of things check out:

http://www.spatialreference.org/ref/epsg/2046/
http://www.spatialreference.org/ref/epsg/2047/
...
http://www.spatialreference.org/ref/epsg/2055/

None of these provide a proj4 definition though, nor can I find one in
PostGIS or via OSR, implying to me that it would be difficult to project
into these projections if using some FOSS Tools. 

I don't think your proj4 style attempt will quite work though, because it
does not provide for the necessary southward orientation. LO is a
south-oriented Transverse Mercator AFAIK

When resolved, this should be contributed to SpatialReference.org, I think

Regards
Graeme

>>> "Andrew de Klerk" <andrew at quartex.co.za> 07/21/09 2:30 PM >>> 
Jeff - It has already asked on this list. I am yet to get a conclusive
definition of what it is from about three different lists. 

I guess it is not as clear cut as it seems. I would have also thought that
with the Quantum training session the other day, we would have our own proj4
definitions sorted out. No?

Andrew

-----Original Message-----
From: africa-bounces at lists.osgeo.org [mailto:africa-bounces at lists.osgeo.org]
On Behalf Of Jeff McKenna
Sent: 21 July 2009 02:08 PM
To: Africa local chapter discussions
Subject: Re: [OSGeo Africa] EPSG 205x

Andrew de Klerk wrote:
> Hi
> 
>  
> 
> Can anyone confirm what the correct Proj4 definition is for our 
> Hartebeesthoek projections. The closest I have come up with is (for 2054):
> 
> +proj=tmerc +ellips=WGS84 +datum=WGS84 +lon_0=31 +k_0=1 +units=m +no_defs
> 
>  
> 
> I have tried on numerous occasions to get someone in the know to say if 
> its wrong or right. I do not know enough about the inner workings of 
> projection systems nor the different options available in the proj4 
> definitions - so if anyone who knows more than me about either of these 
> can help me/us come up with a definite definition this would help.
> 
>  
> 
> Gavin - I know you were looking into this a while ago? Did you get any 
> further?
> 
>  
> 
> Thanks
> 
> Andrew

Andrew,

Maybe you should ask this question on the Proj mailing list 
(http://lists.maptools.org/mailman/listinfo/proj), I bet that is where 
you'll find help.

-jeff

-- 
Jeff McKenna
FOSS4G Consulting and Training Services
http://www.gatewaygeomatics.com/

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