[postgis-users] Using LAT LONG coords with postgis
craig at millerfam.net
Wed Jan 1 19:05:52 PST 2003
Most of what you say is the common modern day understanding of the different
terms, however that understanding is heavily influenced by modern day
applications. In fact, there are numerous serious errors in your summary. I
don't want to critique your efforts here, but if you are considering the
addition to an FAQ it is probably important that it is summarized correctly.
Perhaps a geodesist could provide a summary.
I am not an geodesist, but I believe the following statements are true:
1. Geographic coordinates are coordinates given relative to a
datum/spheroid (E.g. NAD27, NAD83, WGS84, GPRS80).
2. One application of projected coordinates are used to model a sub section
of the earth, however the term and the original application of projected
coordinates was to represent spherical (geographical) coordinates on a flat
surface. Mercator, Robinson, and hundreds of other projections are also
used to represent the entire spherical earth on a flat sheet of paper.
3. Projected coordinates are not always cartesian. E.g. polar projections
do not use cartesian coordinates.
4. It is probably important to note that all projections distort the real
data. A projection is selected based on its properties and how those
properties interact with the intended application of the map. For example,
the Mercator projection was popularized because it does not distort bearings
and is therefore useful when travelling the oceans. It distorts area
heavily (I guarantee you that Antarctica is not that big! :-))
----- Original Message -----
From: "Michael O'Sullivan" <mos at i3sp.com>
To: <postgis-users at postgis.refractions.net>
Sent: January 01, 2003 6:17 PM
Subject: [postgis-users] Using LAT LONG coords with postgis
> I have a few questions on using lat/long data with postgis. I think I
> have the basic gist of things and have a prototype up and running,
> however I would appreciate any feedback on my assumptions. I am willing
> to write up a getting started with LAT/LONG coordinates tutorial and
> submit to postgis doc/FAQ if people think it would be worthwhile.
> LAT/LONG coords are geopgrahic coords, and as such require a geographic
> (as opposed to a projected) coordinate system when being used with
> postgis. Projected coordinate systems are generally used to model a
> sub section of the earth using cartesian coordinates. One of the more
> comonly used projected coordinate systems is the Universal Transverse
> Mercator mapping.
> Given two points in lat/long the length_spheriod function can be used to
> calculate the distance between them. Most coordinate systems seem to be
> based on the GRS80 or WGS84 ellipsoids, which are almost the same -
> having slightly different parameters. GPS uses WGS84.
> What is the accuracy you can expect here, using GRS80 or WGS84? I assume
> issues such as position on earth, distance between two points, and the
> fact that altitude is not taken into account would all impact.
> Also, are there any issues with using the lat/long coords with the &&
> operator and BOX3D object to do bounded by queries? I am currently using
> the BOX3D with a layer of lat/long points to select all points within a
> given radius of another point and it seems to work fine.
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