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Thanks to all who replied to my questions regarding map
projections - you will find below all the responses I
received.
I also asked about downloading my Garmin GPS and include
responses to this.
Many thanks - if I have missed anyone off my apologies - I
stored all the replies in specific mail folders and may
have overlooked a couple but I don't think so.
Lorna
Original request
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I'm currently digitizing (using Cartalinx) contours of
my study area from an orthophoto which is in Gauss Conform
Projection. This is not one of the projections listed as
supported by Cartalinx. However I gather that some
projections are identical to other more common ones e.g.
one of the Gauss is the same as Mercator. So - a few
questions for anyone out there who can answer them please:
My key questions with one set of answers:
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> 1. Does anyone know if Gauss Conform is the same as any
> other projections?
As far as I know it's a cilindrical transverse projection so that it
should be possible to build a UTM reference file suited for it.
(check ellipsoid, datum, central meridian, true and false origins).
> 2. The area I am digitising covers no more than a degree
> but is on a 1:10,000 scale - does the projection even
> impact at this scale?
Of course it does.
> 3. I am currently using a PLANE and KM setting for
> digitising these contours but ultimately want to
> integrate data from GPS and satellite images. My GPS
> operates with LAT/LONG and WDGS 84. Am I creating
> problems for the future or will I be able to transform
> this stuff in IDRISI?
If you will able to get a consistent reference file for your
original data the conversion will not be difficult.
> 4. Can I download tracklogs directly from a Garmin GPS III
> to IDRISI (NMEA format)? I'm currently trying to do it
> using FUGAWI software and its driving me mad by not
> working when its supposed to be fully compatible.
I can't help you. (still I haven't tried to import from my GPS,
may be I'll ask you for help when the time comes)
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The transverse form of the Mercator projection was first described by
Lambert (in the late 1700's), but only in its spherical form. In the early
1800's, Gauss described the ellipsoidal form. Finally, in the early 1900's,
Kruger published formulas for the ellipsoidal version. As a consequence,
the projection, in its ellipsoidal form, is known variously as the
Transverse Mercator, the Gauss Conformal, or the Gauss-Kruger. The latter
term is more popular in Europe while the former is more popular in America.
In IDRISI for DOS Version 4.1 and IDRISI for Windows Versions 1 and 2, you
should specify your projection as "Transverse Mercator". In IDRISI32 you
can also enter "Gauss-Kruger" since it also understands this key word to be
equivalent to Transverse Mercator.
With regards to whether it matters at 1:10,000, the answer is almost always
yes, but sometimes no. It all depends on how close you are to a standard
point or line and the extent of the coverage. You should always take care
with this, if for no other reason that it establishes a basis for
transformations. For example, your GPS information can easily be projected
to the characteristics of your map (or vice versa) if you establish a
proper REF file.
With regards to downloading Garmin GPS III tracks, CartaLinx supplies a
free utility called "Waypoint Plus" that can be used for this purpose --
you should find it on your Version 1.2 CartaLinx CD.
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If it is an issue (and I think it is), why not transform your
orthophoto to the projection and datum you wish with a program such as
the Geographic Transformer or ER Mapper?
The Geographic Transformer may be found at www.bluemarblegeo.com. I
don't have the address for ER Mapper.
Roy Cole,
Geography and Planning Program,
Grand Valley State University,
Allendale, Michigan, USA.
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Don't know anything about a Gauss Conform projection, but I
can tell you
that if you are wanting to combine your contours with data obtained in
another reference system (Lat/Long for instance) then you HAVE to be able to
tell CartaLinx (or IDRISI) what the details of each reference system are.
Therefore, if Cartalinx doesn't support your Gauss Conform projection, you
can't combine data using it with any other data you might have.
The way around this problem is to "change" the reference system which you
are digitising your data in. The smaller the area you are digitisng the
better. If it is the size of a standard 1:10,000 map then you should loose
very little accuracy. It is almost always easiest to change to Lat/Long.
The easiest way is to find a couple of "control points" on your map for
which you know the Lat/Long co-ords. Then simply re-create (i.e. draw
manually) a new co-ordinate grid system based on this. You can then
digitise directly to degrees in Lat/Long rather than metres in the Gauss
Conform.
Hope this helps you. Write back if I haven't been clear enough.
By the way, you would not be able to do anything with data that had "Plane"
as their reference system. Plane is a simple notional orthogonal metres
grid system that cannot be referenced geographically to the curved surface
of the earth. Having said that, it is OK as long as ALL your data use it.
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