| Re: Converting map altitude coordinates (Z) [message #74859] |
Mon, 07 February 2011 08:01  |
mankoff
Messages: 131
Registered: March 2004
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Senior Member |
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On Feb 5, 4:37 pm, David Fanning <n...@dfanning.com> wrote:
> mankoff writes:
>> I can use the MAP_PROJ routines to convert a lat/lon vector from one
>> coordinate system (WGS84) to another (IDL Cylindrical). But if I have
>> a 3rd vector of altitude, how do I convert this?
>
>> The coordinates are currently WGS84 and the altitude is -28 m over the
>> ocean, so I think this is a spheroid issue, and converting to a
>> different system should give 0 m over the ocean. I could just add 28
>> to everything, but I presume there is a more 'official' method.
>
> I don't know the answer to this question, exactly,
> but I do thing your thinking (or maybe only your
> explanation) is a bit fuzzy. WGS84 is a datum, not
> a map projection. (A map projection consists of a
> datum and a data coordinate system.) The MAP_PROJ
> routines will convert from one map projection to
> another, and I presume if there is a datum transformation
> (for example, from WGS84 to a spherical datum) they
> might handle some of that for you, but it isn't
> exposed to you.
>
> I find it a little hard to know what "altitude" would
> mean in terms of a datum transformation. What is the
> altitude measured from? Is it the height above the
> datum, or the height above the geoid (which is different
> from the datum and typically represents the mean sea
> level). You would have to know, I would think, to make
> sense of this question.
>
> If you are looking for a good book to explain all this,
> I *highly* recommend Datums and Map Projections, 2nd Ed.,
> by Jonathan Iliffe and Roger Lott. Very clear, very
> thorough treatment of a confusing and difficult subject.
>
> Cheers,
>
> David
>
> --
> David Fanning, Ph.D.
> Fanning Software Consulting, Inc.
> Coyote's Guide to IDL Programming:http://www.idlcoyote.com/
> Sepore ma de ni thui. ("Perhaps thou speakest truth.")
Hi David,
Thanks for the clarification and suggested reading. Yes my thinking is
a bit fuzzy. I've been doing mapping work for several years now but
there is *always* another layer of complexity to unravel.
Each data point has an elevation (ellipsoid height) and a geographic
lat and lon. This isn't helpful as you mention datum and geoid, but
not ellipsoid. I think the ellipsoid is probably a simple geoid model?
Except you state that geoid is mean sea level, in which case I
shouldn't have values of -28 above this, so perhaps ellipsoid isn't
geoid.
Any hints how to convert from ellipsoid to something where local sea
level is 0 m?
Thanks,
-k.
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| Re: Converting map altitude coordinates (Z) [message #74873 is a reply to message #74859] |
Sat, 05 February 2011 16:37  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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mankoff writes:
> I can use the MAP_PROJ routines to convert a lat/lon vector from one
> coordinate system (WGS84) to another (IDL Cylindrical). But if I have
> a 3rd vector of altitude, how do I convert this?
>
> The coordinates are currently WGS84 and the altitude is -28 m over the
> ocean, so I think this is a spheroid issue, and converting to a
> different system should give 0 m over the ocean. I could just add 28
> to everything, but I presume there is a more 'official' method.
I don't know the answer to this question, exactly,
but I do thing your thinking (or maybe only your
explanation) is a bit fuzzy. WGS84 is a datum, not
a map projection. (A map projection consists of a
datum and a data coordinate system.) The MAP_PROJ
routines will convert from one map projection to
another, and I presume if there is a datum transformation
(for example, from WGS84 to a spherical datum) they
might handle some of that for you, but it isn't
exposed to you.
I find it a little hard to know what "altitude" would
mean in terms of a datum transformation. What is the
altitude measured from? Is it the height above the
datum, or the height above the geoid (which is different
from the datum and typically represents the mean sea
level). You would have to know, I would think, to make
sense of this question.
If you are looking for a good book to explain all this,
I *highly* recommend Datums and Map Projections, 2nd Ed.,
by Jonathan Iliffe and Roger Lott. Very clear, very
thorough treatment of a confusing and difficult subject.
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.idlcoyote.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")
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| Re: Converting map altitude coordinates (Z) [message #74957 is a reply to message #74859] |
Mon, 07 February 2011 08:38 |
David Fanning
Messages: 11724
Registered: August 2001
|
Senior Member |
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mankoff writes:
> Thanks for the clarification and suggested reading. Yes my thinking is
> a bit fuzzy. I've been doing mapping work for several years now but
> there is *always* another layer of complexity to unravel.
>
> Each data point has an elevation (ellipsoid height) and a geographic
> lat and lon. This isn't helpful as you mention datum and geoid, but
> not ellipsoid. I think the ellipsoid is probably a simple geoid model?
> Except you state that geoid is mean sea level, in which case I
> shouldn't have values of -28 above this, so perhaps ellipsoid isn't
> geoid.
>
> Any hints how to convert from ellipsoid to something where local sea
> level is 0 m?
You have me pretty much out on thin ice now. :-)
The geoid is usually taken to be the mean sea level. It looks
more like a wavy ellipse, because sea level varies from point to
point due to underlaying rock, etc. Typically, you choose your
datum (ellipsoid and reference point) to match the geoid at
the chosen location you plan to study. If you look at a
diagram of the geoid and the reference ellipsoid, the geoid
will flow inside and outside the boundaries of the ellipsoid,
but, if you have chosen well, will generally track the ellipsoidal
boundary.
I would guess, in your case, that "altitude" is measured
from the geoid. This would explain how it could have a negative
value with respect to the ellipsoid. (How would an instrument
measure the "height" above some fictional reference ellipsoid!?
It seems to me it has to have some reference to measure against.)
So, I can't really answer your question, because I'm not really
sure what is being measured. But, I presume if "altitude" is measured
to the geoid, then there ought to be some way to reference not to
the WGS84 ellipsoid, but to any ellipsoid you choose to use. In other
words, you might have to calculate the "height" difference between a
WGS84 ellipsoid and a spherical ellipsoid at some specific point, and
then "adjust" the altitude measurement by this difference.
In other words, if the "altitude" at some specific point is -28m from
the geoid to the WGS84 ellipsoid, and the difference at that specific
point between the WGS84 ellipsoid and a spherical ellipsoid is +10m,
then the point should be adjusted to read -38m. I presume this will
have to be done on a point by point basis, since the distance from
the WGS84 datum and the spherical datum will vary continuously and in
a non-linear way.
Something to think about, anyway. :-)
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.idlcoyote.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")
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