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Posted by georger.silva@gmail.com on July 14, 2006, 5:11 pm
Please log in for more thread options since its a small area, and considering the scale, your maximum RMS
acceptable error is: 5
if the error is less then that, and less then 1, dont worry, its a
acceptable map.
of course, if it gets closer to 5, you might have to re cadastrate your
ground control points, or even do another field survey to obtain new
data from gps.
willem van deursen wrote:
> Oh, added to my earlier answer, you have to make sure that your background map
> is well aligned: making sure that your y direction is pointing straight north.
> If you scanned you background map, this is generally not the case and you will
> have to rotate it to make sure this is actually the case. Otherwise you end up
> with relations as
> x = f(lat,lon) and
> y = g(lat,lon)
>
> Willem
>
> willem van deursen wrote:
>
> > Hello GRo,
> >
> > I think you need to consider that your 'scale' (pixels per degree) is
> > different in your x and y direction. Especially if your coordinate
> > system is lat/lon, 1 degree lat is a totally different distance than 1
> > degree lon. So what you propose can and will work only if you develop a
> > relation for your x-direction AND a different relation for your
> > y-direction. (Since your area is small, you don't have to consider the
> > viariation in your x and y relation within your area)
> >
> > Willem
> >
> > GRo wrote:
> >
> >> Hi, hopefully someone can understand my question, and help.
> >>
> >> Is there any reason why I can't do this .......
> >>
> >> ......take raster maps of small geographic areas (20km x 20km) with
> >> cartesian coordinate systems - i.e., UK Ordnance Survey 1:10000
> >> Landplan series - and consider that WGS84 latitude/longitude from a
> >> GPS receiver can linearly be converted to pixel positions on that map
> >> image, provided the map image has been accurately geo-referenced using
> >> WGS84 lat/long to pixel referencing?
> >>
> >> I ask because this is what I'm trying to do ;-)
> >> But I'm seeing significant inaccuracies which vary in both x and y
> >> axis. My *suspicion* is that the inaccuracies are dominantly due to
> >> inaccuracies in my georeference "calibration" measurements (which I'm
> >> taking with a handheld, retail-quality GPS receiver).
> >>
> >> If I use a much higher quality receiver, for example, with WAAS
> >> enabled, will I see my results improve, or am I ignoring some subtely
> >> of GIS systems which I need to take heed of, for example, something to
> >> do with coordinate transformations between different datums, etc.
> >>
> >> Any comment will be helpful, I'm sure.
> >>
> >> Regards,
> >>
> >> GRo
> >>
> >
>
> --
> Willem van Deursen, The Netherlands
> wvandeursen_nospam@nospam_carthago.nl
> replace _nospam@nospam_ for @ to get a valid email address
> www.carthago.nl
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