GPS risks

[Una Smith]

Global positioning systems are becoming very popular as a method to assign
map coordinates to collecting localities.  However, until reading the note
below, I was not aware of the sometimes very large discrepancy between GPS
coordinates and local map coordinates.  Thus, it may be important to record
in collections databases the precise source of locality coordinates.  E.g.,
if read from a map (what map?) or captured in the field with a GPS.

        Una Smith

>RISKS-LIST: Risks-Forum Digest  Monday 26 January 1998  Volume 19 : Issue 57
>
>   FORUM ON RISKS TO THE PUBLIC IN COMPUTERS AND RELATED SYSTEMS (comp.risks)
>   ACM Committee on Computers and Public Policy, Peter G. Neumann, moderator
>
[...]
>
>Date: Mon, 26 Jan 1998 13:39:00 -0800
>From: Ron Crandall <xrc at isi.com>
>Subject: GPS position accuracy and EGPWS
>
>The recent note on EGPWS prompted me to write this note.  I don't remember
>having seen a discussion in RISKS about the GPS reference datum problem that
>can be a significant hazard.
>
>In short, the GPS system uses a model of the Earths shape (an ellipsoid)
>that is suitable for the entire Earth.  Almost all printed maps, and quite
>likely many of the electronic maps derived from them, use an assortment of
>ellipsoids that were typically optimized for a country or region.  The GPS
>position when plotted on one of these maps can be off by a considerable
>amount, as much as seven miles in some Pacific areas.  The half mile
>discrepancy in the Caribbean has already led to several boat wrecks when
>they tried to navigate a narrow passage in poor visibility using GPS data.
>
>Considerable detail is available in yachting publications (e.g., the current
>Ocean Navigator magazine), but the EGPWS article shows a way in which
>aviators might be using unsurveyed approaches that would be subject to this
>risk.
>
>Ron