Q1.
I recently downloaded your Geoid Height Calculator (installed and working fine) but it now
makes me wonder how the ellipsoid relates to sea level. Putting it another way, suppose I
have GPS coordinates and heights that are based on the WGS84 datum and ellipsoid. Is there
a way to determine the amount that the ellipsoid height is either above or below sea level
in the general area of the measured point?
Thanks,
Scott
A. Hello Scott . . .
Essentially, that's what the geoid height is. Some prefer the term
geoid separation. Most use geoid height.
The geoid is the new modern replacement for sea level. Given that
the tides are a 20 (??) year cycle and there is the normal swell, plus weather
disturbances, sea level is a nice reference, but a very difficult one to get a hold of for
precise measurements. Enter the geoid.
The shape of the surface of the earth is stated to be the form of a
equi-potential gravitational surface. That is, the locus (i.e. collection) of all
points at which the force of gravity has the same specific magnitude value. The
specific magnitude value is adjusted so that the resulting surface
"approximates", or best fits, sea level. This final surface is then called the
geoid.
What most folks refer to as elevation is now referred to as orthometric
height. This is height above the geoid. There is also a thing called
ellipsoidal height. This is height above the ellipsoid. The difference between
these two is the geoid height.
GPS units, prior to any processing by the data collector, deliver
ellipsoid heights. The orthometric height, or sea level elevation, is obtained by
subtracting the geoid height. (Note that geoid height is often
negative, which must be observed.)
Hope this helps.
Norm
Q2. I tried your
(Forward/Inverse) program and got some weird results on the 1st attempt. I later went to
your FTP site and D/L'd Inverse.zip. I tried that program and it gave me the correct
result. Is there some flakiness in your program?
Here are the coordinates:(DDD-MM-SS)
Station 1
N36-53-40
E007-38-24
Station 2
N36-41-56
E005-39-33
With your program, I get the result of the azimuth from Station 1 to
Station 2 as 96º 24' 15.96"W. With my old DOS program, and the Inverse program
from your FTP site, I get the results of 263º 33' 58".
I have tried it with and without the leading zeros in the longitude.
Any clues as to what the problem might be?
Thanks and regards,
Dave
A. The result produced by our Windows version
is "correct". I use quotes, as when I wrote the program, I did not know
that standard geodetic convention requires azimuth to be written in the zero to 360 range.
If you look at the result from our windows version, the result looks something
like:
96 24 15.96W
where the trailing W is to indicate degrees west of true north, i.e. a
negative number. If you convert this value to its positive equivalent, you get the
263 33 58 which you are expecting.
I have been wanting to update this program for some time now. It
just hasn't made it to the top of the list yet.
I do appreciate your bringing this to my attention. I'm a
"Casual Cartographer" and I don't know everything. All I do know is that
I'll never know everything. So, again, thanks for the help.
Norm
Readers Respond
I enjoyed reading the CC. Regarding the length of the vara, for
American users in Texas and California, the vara has been standardized as 33 1/3
inches and 33 inches respectively.
Dana Lattin
***************
I have a suggestion for Norm for a "Freebie of the Month". I
think it would be handy to have a utility where you could type in the City and State and
have the State Plane Zone returned to you. This routine could be similar to the
Telephone Area Code Freebie.
As an engineering consulting firm working in just about every State
within the U.S., I think this would be very helpful.
Michael Fate
Boyle Engineering Corp
Thanks Michael, We took your advice! Check out the State Plane Zone Locator
Do you have a question for Norm? Just
ask him.
Check out The Casual Cartographer Archives for more Ask Norm
Q&A.
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