FW: [OSGeo Africa] EPSG 205x
Calle Hedberg
chedberg at telkomsa.net
Wed Jul 22 10:30:18 EDT 2009
Walter,
Not accepting established international standards in an area might not be
wise and is nowadays a bit of a pain, yes, but in all fairness: It had
nothing to do with stupidity or laziness.
Historically - and we are talking 30's, 40's, and 50's here - it was more a
result of resistance against automatically adopting projections and "world
views" developed for and by Western powers. UTM, for instance, was
originally designed by the US Army in the 40's, it then became a NATO
standard and then a de facto international standard. In South Africa this
was resisted, I guess, partially for scientific reasons (SA felt 2 deg zones
were more accurate and usable than 6 deg zones, as far as I can recall) and
partially due to the "laager" mentality of the regime - making its mark
scientifically (ref Chris Barnaard and others later) became one way of
"proving" the benefits of white supremacy.
While most of that is history now, there is of course a more general
challenge for cartographers and GIS professionals working in the south: we
have a responsibility for addressing the imbalances in "world views" that is
inherent in the language, methods, and standards used in the field. Just
think of how dominant Mercator maps are for most people's view of the world:
Countries in the north are always on top, and they are grossly inflated in
terms of size compared to countries in the south (which in general is closer
to the equator).
When lecturing, I usually start with a more correct map of the world turned
"upside-down" - and then crack some jokes about "now we are on top and the
yankees and scandihooligans are hanging by their fingernails trying not to
fall off..." to get people to reflect on why we always tend to think of the
world in a certain way.
Because scientific and technological hegemony always come with a price-tag,
either directly (ESRI pricing...) or indirectly, like when somebody else
ends up determining what we can do through their own (selfish, some times)
decisions on satellite sensors, GPS accuracy, data release policies, GIS
modelling, and so forth.
So while those SA cartographers back in the 30's and 40's went their own way
partially for reasons we don't accept, they at least understood that the
countries in the south need to develop structures and standards that fit our
own needs... Which for me is one powerful reason for using more Free and
Open Source technologies, by the way
Best regards
Calle
Cape Town
-----Original Message-----
From: africa-bounces at lists.osgeo.org [mailto:africa-bounces at lists.osgeo.org]
On Behalf Of Walter Smit
Sent: 22 July 2009 09:22 AM
To: 'Africa local chapter discussions'
Subject: RE: [OSGeo Africa] EPSG 205x
Hi Andrew,
The problem is that many people in South Africa follow international
practice and use the "LO system" NAME for "normal" Mercator projections (not
south facing). This leads to a LOT of confusion.
This "South African LO" projection has always been stupid in my opinion (and
many other people I know). Just because some surveyor was too lazy to write
long numbers and negative signs many many years ago does not mean we should
use it. I for one never use it and advise other people to rather use the
"correct" version of tmerc.
In other words: your proj4 definition is correct (for normal Mercator
projection) - the south facing LO projection is just wrong. If you really
need to define the upside down LO projection you can just set the scale to
-1 in ESRI products (which I believe run on GDAL/OGR??). Unfortunately this
doesn't seem to work in Quantum. Dammit.
Just my 2 cents (and a little ranting) :-)
Regards
Walter
-----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 04:31 PM
To: 'Africa local chapter discussions'
Subject: RE: [OSGeo Africa] EPSG 205x
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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