Just for our reference, how much elevation change will you be working with? What is the highest elevation?
James
This series of papers will answer a lot of questions about heights. It may also raise some others, but it's really good nerdland reading.
http://digitalcommons.uconn.edu/cgi/viewcontent.cgi?article=1001&context=thmeyer_articles
http://digitalcommons.uconn.edu/thmeyer_articles/3/
http://digitalcommons.uconn.edu/cgi/viewcontent.cgi?article=1001&context=nrme_articles
http://digitalcommons.uconn.edu/cgi/viewcontent.cgi?article=1004&context=nrme_articles
NCSpiralGuy, post: 403246, member: 12287 wrote: At 39å¡N 22å¡E, the geoid is 33.3 or 33.2 meters above the ellipsoid, depending on which geoid model you use (those are two different NGA files, the precise numbers are 33.214 and 33.302, and the conversion to boldatni may have changed it by a millimeter or two). You'll probably want to use a geoid file published by the Greek government. The elevation factor at elevation 0 (which means geoid level, which may differ a few centimeters from sea level) is 0.99999479 using the more recent and finer NGA model. So if you forget to enter a geoid file and you take GPS readings converted to Greek state plane coordinates and ground measurements with gun and prism pole, you will be off by 5 ppm converting between them.
Where did youi get that? Is there somewhere i can put my grid coords and see geoid seperation and elevation factor?
http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/egm08_wgs84.html is where to get the recent and finer geoid file. The old one is a 15-minute grid. I haven't tried running the Fortran programs; they don't come with Makefiles, configure scripts, or CMakeLists files.
The program I used is Bezitopo ( http://bezitopo.org ). You'll have to get the latest from Github to produce gsf files; I just got that working this morning. The 1-minute NGA file is almost a gigabyte, and convertgeoid makes two other arrays the same size, so if you have 4 GB RAM it'll take forever, and if you have 8 GB RAM and a GUI like my laptop, the load average will shoot up for a while. You can tell it to write a file containing all of Greece or just your island.
This is a run of Bezitopo with my location (which I've redacted but I am in North Carolina):
? setlunit m
? geoid g2012b.bol
read g2012b.bol
? factor
LatLong> (my location)
Elev> 340
Geoid is -31.691 m above ellipsoid
Average radius of curvature is 6371084.711 m
Elevation factor is .9999516104594695
LatLong>
? geoid Und1.bol
read Und1.bol
? factor
LatLong> (my location)
Elev> 340
Geoid is -32.753 m above ellipsoid
Average radius of curvature is 6371084.711 m
Elevation factor is .9999517770790578
LatLong>
? exit
Und1 is the NGA file I linked to above; g2012b is the official USA geoid. They differ by over a meter here. The Alaska and Lower 48 files of g2012b disagree in the overlap (convertgeoid says something about Kitimat for this reason). As I said, you want to get the official Greek geoid.
