Re: MAP DIFFERENCES
- From: ed@xxxxxxxxxxxx
- Date: Wed, 01 Aug 2007 12:34:40 -0700
On Jul 25, 9:25 pm, bobbrooks...@xxxxxxxxx wrote:
I remembered that there was a survey marker on the peak of a small
mountain in my area so I went up tonight and found it. Unfortunately
time has worn away all the information. I had an interesting
exsperience though. I usedTopoZonecoordinates for the peak and wound
up about 100 feet lower down the side of the mountain deep in a patch
of juniper before zeroing out. I then went back to the marker which I
assume is the very top and put in a waypoint, averaged it, went to MAP
and found that my waypoint was 61 feet from the Mapsource symbol for
the peak. When I got home I put the coordinates into Terrain
Navigator's USGS map which showed the field waypoint (76CSx) right on
the money. Any thoughts on why all three programs are so different?
Especially the Topo Zone which was way off.
Bob -
I am always puzzled by users who have questions about TopoZone and
seem to ask everyone EXCEPT us <g>. A quick email to
info@xxxxxxxxxxxx will give you all the help we can provide.
Unfortunately you don't provide enough information for me to do
anything right now; "TopoZone coordinates" don't tell me how you got
those coordinates, etc. Dan's comments on GNIS data are probably
relevant here, but if you use TopoZone's place name search to look for
a named summit and then go to the topo map, you should be able to see
whether the red coordinate target symbol is on the survey marker
location or not. Rounding errors as Dan reports are quite common in
the GNIS database we (and just about everyone else) uses.
For example, if I search for Mount Washington, NH, I get here:
http://www.topozone.com/map.asp?lat=44.27056&lon=-71.30472&datum=nad27&u=4&layer=DRG&s=24
and while I'm obviously on the summit of Mount Washington, the "BM
1916.6" mark is obviously some distance away. Clicking on it takes me
here:
http://www.topozone.com/map.asp?lat=44.27039&lon=-71.30380&datum=nad27&u=4&layer=DRG&s=24
which is a difference of 0.00017 degrees in latitude and 0.00092
degrees in longitude (all NAD27 datum).
If you can describe exactly what summit and marker you're describing,
we 'd be happy to investigate further.
- Ed
Ed McNierney
Chief Mapmaker, TopoZone.com
.
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