Since all spatial data are about describing a specific location on a specific planet, usually earth, it would seem that the best way conceptually to store data is with respect to a single easily defined reference point such as the gravitational center of the planet. Any location could then be measured with three values. x,y like latitude and longitude, and z a distance measure from the reference point along a ray.
Projections such as utm, etc, are about how to convert the 3-d data described above into 2-d with a minimum of distortion. Given the speed of modern computers this conversion process ought to be increasingly easy to do on the fly, as needed.
The reason I raise this question is to ask the experts whether it would make sense (for 7.x) to think of a single standard way of storing data in grass and then all operations would do the conversions as necessary? There are (at least) two advantages of this. One is standardization of data storage in a form that is closest to a true representation of the real world. A second is to reduce the potential for confusion/mistakes when data are shared and the metadata are not, or are inadequate. I am continually getting access to data where the units are not clearly defined. But even if they are defined say as some utm coordinate, there must be some error in measurement built in.
Just some thoughts on a Sunday morning.
Regards,
JErry
Gerald Nelson
Professor, Dept. of Agricultural and Consumer Economics
University of Illinois, Urbana-Champaign
office: 217-333-6465
cell: 217-390-7888
315 Mumford Hall
1301 W. Gregory
Urbana, IL 61801
Apparently you are not working often in polar areas, so such problem
of spherical coordinates as singularities is not obvious.
Gravitational center, to my understanding, is not an easily defined
point either. Also, from ArcGIS experience, on-the-fly reprojection is
quite slow process even using most powerful desktop.
Boris
On Dec 30, 2007 8:12 AM, Gerald Nelson <gnelson@uiuc.edu> wrote:
Since all spatial data are about describing a specific location on a specific planet, usually earth, it would seem that the best way conceptually to store data is with respect to a single easily defined reference point such as the gravitational center of the planet. Any location could then be measured with three values. x,y like latitude and longitude, and z a distance measure from the reference point along a ray.
Projections such as utm, etc, are about how to convert the 3-d data described above into 2-d with a minimum of distortion. Given the speed of modern computers this conversion process ought to be increasingly easy to do on the fly, as needed.
The reason I raise this question is to ask the experts whether it would make sense (for 7.x) to think of a single standard way of storing data in grass and then all operations would do the conversions as necessary? There are (at least) two advantages of this. One is standardization of data storage in a form that is closest to a true representation of the real world. A second is to reduce the potential for confusion/mistakes when data are shared and the metadata are not, or are inadequate. I am continually getting access to data where the units are not clearly defined. But even if they are defined say as some utm coordinate, there must be some error in measurement built in.
Just some thoughts on a Sunday morning.
Regards,
JErry
Gerald Nelson
Professor, Dept. of Agricultural and Consumer Economics
University of Illinois, Urbana-Champaign
office: 217-333-6465
cell: 217-390-7888
315 Mumford Hall
1301 W. Gregory
Urbana, IL 61801
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Gerald Nelson wrote:
Since all spatial data are about describing a specific location on a
specific planet, usually earth, it would seem that the best way
conceptually to store data is with respect to a single easily defined
reference point such as the gravitational center of the planet. Any
location could then be measured with three values. x,y like latitude
and longitude, and z a distance measure from the reference point along
a ray.
Projections such as utm, etc, are about how to convert the 3-d data
described above into 2-d with a minimum of distortion. Given the speed
of modern computers this conversion process ought to be increasingly
easy to do on the fly, as needed.
The reason I raise this question is to ask the experts whether it
would make sense (for 7.x) to think of a single standard way of
storing data in grass and then all operations would do the conversions
as necessary? There are (at least) two advantages of this. One is
standardization of data storage in a form that is closest to a true
representation of the real world. A second is to reduce the potential
for confusion/mistakes when data are shared and the metadata are not,
or are inadequate. I am continually getting access to data where the
units are not clearly defined. But even if they are defined say as
some utm coordinate, there must be some error in measurement built in.
Apart from wasting CPU time, conversion introduces error. Applying a
non-affine transformation to a regular grid (i.e. raster) doesn't
result in a regular grid. Applying a non-affine transformation to a
straight line doesn't result in a straight line. Any spatial
measurement which is constant for the original data (e.g. maximum
spatial error) will cease to be constant if the data is projected.
All things considered, the optimum form in which to store the data is
the form in which the user chooses to store it. There will always be
factors of which the user is aware but the software isn't.
--
Glynn Clements <glynn@gclements.plus.com>