[San-Diego-pm] Meeting today (Monday, March 13th, 2006)
tkil at scrye.com
Tue Mar 14 15:50:52 PST 2006
>>>>> "Todd" == Todd Rockhold <tdrsubs_2003 at yahoo.com> writes:
Todd> Just some idle speculation which may well be completely wrong
Todd> because I don't know details of Google Maps, GPS computations,
Todd> or Joel's observed discrepancies.
Todd> I have heard that for accuracy GPS equipment does incorporate an
Todd> oblate spheroid model of the earth. Haven't heard one way or
Todd> the other about Google Maps, but if it incorporates a shpherical
Todd> model of the earth that could account for discrepancies. But
Todd> would those match Joel's observations?
The concept of a "datum" in this sense is exactly what you're talking
about: what model for the earth's surface is in use, and what are its
The Wikipedia link explains it pretty well:
The difference in co-ordinates between data is commonly referred to
as datum shift. The datum shift between two particular datums can
vary from one place to another within one country or region, and
can be anything from zero to hundreds of metres (or several kilo-
metres for some remote islands). The North Pole, South Pole and
Equator may be assumed to be in different positions on different
datums, so True North may be very slightly different. Different
datums use different estimates for the precise shape and size of
the Earth (reference ellipsoids).
The difference between WGS84 and OSGB36 is up to 140 metres / 450
feet, which for some navigational purposes is an insignificant
error. For most applications, such as surveying and dive site
location for SCUBA divers, 140 metres is an unacceptably large
The main reason that there are a number of datums is that before
the advent of GPS positioning, national map making organisations
did not have a common surveying reference point and only produced
maps for their locality.
(The word "datums" sends chills up my spine, really.)
Most of the maps from the USGS for years have been based off the North
American Datum from 1927 ("NAD27"). GPS is based off the World
Geodetic System 1984 ("WGS84"). More Wikipedia goodness:
The WGS 84 orignally used the GRS 80 reference ellipsoid, but has
undergone some minor refinements in later editions since its
initial publication. Most of these refinements are important for
high-precision orbital calculations for satellites, and have little
practical effect on typical topographical uses. The following table
lists the primary ellispoid parameters.
[chart omitted; the only difference is the inverse flattening,
which for GRS80 is 298.257 222 101, while for WGS84 it is
298.257 223 563]
For the very small difference in the flattening results a -- very
theoretical -- difference of 0.1 millimetre in the polar axis.
Longitudes on WGS84
The longitude positions on WGS84 agree with those on the older
North American Datum 1927 at roughly 85 longitude west, in the
east-central USA. By contrast, the zero meridian of WGS84 is about
100 metres east of the "traditional" Prime Meridian at Greenwich,
So the difference between NAD27 (USGS topo maps) and WGS84 (GPS) could
easily be in the dozens of meters out this way.
The simplest way to do this is probably to take GPS readings at
obvious map features (peaks, road intersections, etc) and compare that
to what you get by reading the map for the same features. If you do
the feature observations in an small area (within a 50mi radius, say)
surrounding your intended target, you should be able to come up with a
simple lat+long translation that you can use to convert map readings
to gps settings or v-v.
(What's interesting is that Joel indicated that his GPS matched his
topo maps, which means that his GPS is likely already set to read
NAD27; to see what Google is using, he can either just ask, or perform
the map-vs-secondary-source experiment above while sitting at home:
find features on the topos in the area of his intended target, find
those same features on google maps, and see if there's a consistent
pattern. I have noticed that google's satellite photos, at least,
tend to be less-accurately aligned with geopolitical / man-made
features (such as roads) when you get out into the sticks.)
Adding to the confusion, it looks like UTM *used to be* based on the
European Datum 1950 ("ED50"). Wikipedia takes it once more:
The Universal Transverse Mercator coordinate system was developed
by the United States Army in 1947. The system was based on an
ellipsoidal model of the Earth. For areas within the conterminous
United States, the Clarke 1866 ellipsoid was used. For the
remaining areas of the Earth, including Hawaii, the International
Ellipsoid was used. Currently, the WGS84 ellipsoid is used as the
underlying model of the Earth in the UTM coordinate system.
So to be clear, and depending on the era in which the maps were
generated (and possibly what area of the earth it was generated for),
you have to specify UTM as well as the datum in use, so: "UTM ED50",
"UTM WGS84". The latter is what the US is trying to get all their
maps onto, but it takes time to get there.
And for the really juicy bits:
Why yes, I am a map geek, why do you ask?
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