CompuServe Messages

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