Re: GPSMap 60cs Altimeter calibration


"Roy Lewallen" <w7el@xxxxxxxxx> wrote in message
news:11s4jgi46prebc7@xxxxxxxxxxxxxxxxxxxxx
> Roy wrote:
>> . . .
>> I once did something similar to what you did. I tried it at two
>> different survey markers and got very different results at each
>> location. Using a GIII+ (not WAAS capable), I marked 10 waypoints over
>> a period of about 3 months on different dates and times at each
>> location, holding the receiver at arm's length about chest high
>> directly over the point, as close as I could by eye. At one location,
>> I got exactly what I would have expected. The scatter of the waypoints
>> was roughly centered on the coordinates of the survey marker (the
>> average of the northings and eastings were within 2 feet and 1 feet
>> respectively of the coordinates) and all 10 of the waypoints were
>> within 10 meters of the marker coordinates. At the other location,
>> the average of the northings was 27 feet north of the marker northing,
>> and the average of the eastings was 14 feet east of the marker easting.
>> In addition, 5 of the waypoints were more than 10 meters from the
>> coordinates of the marker. Obviously, the results at the second
>> location not only had more scatter, but there was a definite skew
>> toward the northeast. I rechecked the coordinates I was using for the
>> survey marker and am confident they were correct. The location where
>> the poorer results occurred has a reasonably good view of the sky.
>> There are no buildings taller than three stories within a mile of the
>> place. I still don't really know why I got so much better results at
>> one location than the other; but it seems that there was *something*
>> about one of the locations that was throwing things off. There is a
>> single metal utility pole about 30 feet from the survey marker. It's
>> hard for me to imagine that something so thin could have thrown my
>> results off that much, but there isn't anything else I can attribute
>> it to.
>
> How about the possibility that the marker isn't exactly where it says? I
> was under the impression that most of those old markers were placed by
> folks surveying with physical chains and the like. They did a remarkable
> and almost unbelievably accurate job given the technology they had. But
> how close can we really expect the markers to be?
>
> I recall using USGS maps for backpacking in Colorado where there were thin
> pie-shaped slices of land which didn't belong to any coordinate system --
> the surveys coming from the east and west mismatched a bit, leaving slices
> with no official location. Again, the accuracy was amazing under the
> circumstances. But nothing like we take for granted today.
>
> Or am I wrong, and have the locations of all the old markers been checked
> with a high-accuracy GPS system?
>
> Roy Lewallen

Of course, you have to be sure that the datum used to display the
coordinates on the GPS is the same as the one used to identify the location
of the "old marker." I bet it wasn't inscribed using WGS84 back in 1883 (or
whatever). Alos - my greatgrandfather was involved in a court battle about
a "gore" - the name for those bits and pieces of land mentioned above that
seem to be "unowned" or the result of two surveys not meeting mathematically
on a co-incidental boundary. He argued that some land alledged to belong to
the state actually belonged to him since he owned to the county line, and
the line (according to the surveyors own report) didn't quite agree to the
next county's independent survey of the same line. he lost the court case.
Many many years later, when survey quality GPS and GIS "came to town", it
was discovered that now ancient errors in control points and survey
descriptions around town had everyone actually owning each other's property
by description. That took a while to straighten out!


.

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