GPS-Trimble Scout
2 messages in this thread
Rob —
I haven't been able to to post a message here for a while. Your lesson on
datums raiseth a few questions. I read somewhere that the ellipsoid is caused
by the rotation of the earth and the distance from pole to pole is about 1/300
shorter than the diameter at the equator. If I understand what you are saying,
the earth doesn't have a smooth surface even if you remove the land masses so
the geoid is really a lumpy ellipsoid. How much does the geoid vary from a
perfect ellipsoid in elevation? Are there many lumps or just a few and are they
large like say the size of the US or are they smaller than that? Did we
discover the lumps with satellites or did we know about them before? It seems
to me, the bottom line is that the distance between two points as measured by
traditional survey methods would be different than measurments taken from
satellites since the satellite measurments woudln't have to take into account
going over the lumps. Also, the difference between the two measurments would be
different at varios locations on the Earth. Am I on the right track?
My main interest in all this is trying to convert locations on old USGS topo
maps (surveyed in the 1880's) to newer topos. All the old maps say is "Datum is
mean sea level".
— Bill
Detailed reply to follow, Bill ( along with part 2 of the lesson!). I've been
tied up with work pressure for the last few days. However, you are on the right
track. The WGS 84 ellipsoid can vary by up to +/- 100m and then some from the
geoid at various places.
The " Datum is Mean Sea Level" on old maps is not related to ellipsoids at all.
To confuse matters even more, datums can be "horizontal" or "vertical" ( ie, to
do with heights). Geodetic datums such as WGS 84 define 3 dimensional
positions, whereas a Chart Datum or Tidal Datum ( basically anything relating
to sea levels as a datum definition) is defining only heights. ie, on your old
topo maps, the "MSL" datum simply defines the zero point from which all those
spot heights have been levelled in. The lat and long will have been defined
according to an astrogeodetic datum of the type I'm half way through
explaining. It's just that on old maps there was no need to define this, as
there was only one datum in use at a time, and no need to move between them.
More to come…..
Rob.