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| GeoCommunity Mailing List |
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| Subject: | Re: GISList: true v. magnetic North in GPS |
| Date: |
02/24/2003 07:20:58 PM |
| From: |
Ilya Fischhoff |
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hi,
Since I sent my first message, the GIS person at my field station in Kenya has
made progress on the problem. He used DNR Garmin (an Arcview extension that is
a free version of Garmin's Mapquest) to download points as a text file, rather
than as a shapefile. Then he used an Arcview reprojection extension to
reprojected the points from WGS 84 to Arc 1960. (The GPS had been set to Arc
1960, but he had a hunch that either the GPS or Garmin DNR were really using WGS
84). This reprojection resulted in points more closely matching the 1960s-era
topos. One question that others have raised is whether the GPS really used WGS
84 no matter what one asks it to do. But the person I talked to at Garmin said
no to this idea. So, we haven't figured out yet whether it's a problem with DNR
Garmin or the GPS itself, but it does seem to be a datum problem.
Does anyone have experience working with ASTER images and know how accurately
they are georeferenced and how much variation there is among images taken in the
same place? The one I'm looking at is about 100m off relative to an aerial
photo that was itself georeferenced using this 1960 topo. So it's hard to tell
whether the source or the ASTER image is off. I'm using ERDAS and NASA's ASTER
reprojection tool. thanks!
ilya
Richard Ash wrote:
> I would concur with Pete that this setting should not effect positioning.
> It would effect the azimuth readings you receive while navigating.
>
> If ALL (versus a percentage being anomalies) the points are consistently off
> the same distance and direction, the problem is most likely a mismatch
> between coordinate system/datum of the GPS coordinates and your basemap
> layers. This trick is finding where the mismatch originates.
>
> Note, that the projection settings on many GPS units only will determine how
> the data is viewed. The data may actually be stored in a default coordinate
> system/datum - typically Lat/Lon, WGS84.
>
> Good luck.
>
> Richard Ash
> GPS Consultant
> _______________________
> Products, Consulting and Training for GPS/GIS/SURVEY
> Specializing in GPS and Laser Range Finder Technology
>
> Global Mapping Technology
> 1107 NW Oak Avenue
> Corvallis, OR 97330
>
> email: <gmtgps@peak.org>
>
> ----- Original Message -----
> From: "Pete Russell" <wmrussel@excite.com>
> To: <ifischho@Princeton.EDU>: <gislist@geocomm.com>
> Sent: Monday, February 24, 2003 4:10 PM
> Subject: RE: GISList: true v. magnetic North in GPS
>
> >
> > Hi Ilya,
> >
> > I can't answer all your questions about your error, but declination should
> not have any effect on the accuracy of your points. Let me know if you hear
> a differing opinion.
> >
> > Cheers,
> >
> > Pete
> >
> > ======================================
> > Forester
> > Russell Precision Mapping Inc
> > GIS, GPS and Timber Management
> > Based in Richmond, VA and Tuskegee, AL
> > Office: (804) 320-2254
> > Mobile: (804) 356-MAPS
> > ======================================
> >
> > ---
> >
> > Hello,
> > I am a grad student in ecology at Princeton. My thesis is on zebra
> > movement and cognition. My field site is in Laikipia District, Kenya,
> > slightly (<0.5 degree) north of the equator. My GPS data from last
> > summer, collected with a Garmin 76, have errors of several hundred
> > meters, relative to existing topo maps produced by the British army. I
> > think the error is consistent in direction and distance, but I don't
> > have enough points for comparison to be sure yet. I'd like to figure
> > out these errors prior to using my georeferenced habitat data for
> > analyses of ASTER images. I collected my GPS data in Clarke 1880
> > spheroid, Arc1960 datums, the same projection as the topo maps. I am
> > not sure whether I had the GPS set for true North or magnetic North when
> > I collected the data last summer, but should be able to find this out
> > soon with help of someone in Kenya. I'm writing to ask if there exist
> > lookup tables for converting GPS data collected with one declination to
> > the coordinates for the other North reference, for a given lat/lon
> > location. Is it just a linear transformation? Thanks very much for any
> > input. cheers,
> > Ilya
> >
> >
> >
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