Subject: how many model earths & how much do they vary
Date: May 05, 2001 @ 20:03
Author: michael donner (michael donner <m@...>)
Prev Post in Topic Next [All Posts]
Prev Post in Time Next
>
>Michael:
>> can you also say which of the 3 choices gps survey technology adopts
>> & whether this points to a most consensual usage
>>
>> or is it equally at home geocoordinating in all 3 versions
>
>The basis for GPS is the WGS84 ellipsoid. But my (rather elderly) GPS
>receiver provides conversion to many local mapping systems, based on
>many locally-defined best-fit ellipsoids.
>Traditionally, horizontal mapping has been done using an ellipsoid
>model (because the sums are easier to do than if you treat the Earth
>in all its lumpy reality), but vertical positioning must always be
>referred to the geoid, since we're interested in height above mean
>sea level. In the old days, this was all done with tide gauges and
>sight lines.
>So my old GPS gives horizontal data relative to the ellipsoid, which
>match the mapping nicely, but also *heights* relative to the
>ellipsoid, which can be adrift by several metres from MSL. There's no
>reason in principle why the geoid model couldn't be built into a GPS
>receiver (it's basically just a contour map of MSL relative to the
>ellipsoid) although it's intensive in memory and calculation, I would
>guess that such things do exist, but I haven't seen them.
>
>So: ellipsoid maths is sufficiently simple that there's no reason to
>use the spherical approximation. It also transfers GPS data
>accurately to the horizontal detail of a map, which is what most
>people want for navigation. But for accurate height surveying
>information you need a geoid model built into your GPS receiver
>software.
>
>Grant
>
> Yahoo! Groups Sponsor
><http://rd.yahoo.com/M=190462.1393721.2979173.2/D=egroupmail/S=1700126166:N/A=55
>1014/?http://www.debticated.com target="_top"> Your use of Yahoo!
>Groups is subject to the <http://docs.yahoo.com/info/terms/> Yahoo! Terms
>of Service.