[Gdal-dev] Re: Re: how to extract data from OGR

[L Pool]

Thanks Frank,

I figured as much - its just an offset to your own coordinate reference
system, right?

Any my teachers use to tell me I wouldn't amount to anything... Geography is
easy cheesy!

Cheers,
Louis.

-----Original Message-----
From: gdal-dev-admin at remotesensing.org
[mailto:gdal-dev-admin at remotesensing.org]On Behalf Of Frank Warmerdam
Sent: 20 May 2004 07:18
To: gdal-dev at remotesensing.org
Subject: Re: [Gdal-dev] Re: Re: how to extract data from OGR


L Pool wrote:
> Ok. Now for something totally unrelated - this is a question for pretty
much
> anyone who can help me out with a little bit of knowledge on Geographic
> Projections in GDAL.
>
> I've written a few Geographic conversions for certain Projection
> Algorithms - but the maths that I wrote them from does not have parameters
> for "False Eastings" and "Fast Northings" which the OGRSpatialReference
> class definitely has. E.g.
>
> Albers Conic Equal Area:
>
> OGRErr OGRSpatialReference::SetACEA  (  double    dfStdP1,
>   double    dfStdP2,
>   double    dfCenterLat,
>   double    dfCenterLong,
>   double    dfFalseEasting,
>   double    dfFalseNorthing
>  )
>
> you have your Standard Parallels and the Center Lat/Long pair (which is
all
> I use in my projection algorithm) - but where do the False Easting and
False
> Northing params come from? I know what an Easting and a Northing is - I
> don't know how it fits into the projection algorithm tho. Does it have to
do
> with your own Cartesian reference?

Louis,

False eastings and northings are just extra offsets added to the projected
coordinates.  This is normally done to ensure that all numbers in the area
of interest will be positive to avoid "sign issues".  For instance, UTM
coordinate systems have a false easting of 500000.  Normally after
converting
a location to transverse mercator any location west of the central meridian
would be negative.  But by adding 500000 meters, the designers of the
projection
ensure that all eastings will be positive within the area of the zone
itself.

The implementation is simple, and it only exists to make reading and
transmiting map coordinates (by humans) less error prone.

Best regards,
--
---------------------------------------+------------------------------------
--
I set the clouds in motion - turn up   | Frank Warmerdam,
warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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