[postgis-users] Ellipses and WGS84

[Bruce Rindahl]

The example creates an ellipse directly in a projected coordinate 
space.  What you need is the difference between latitude and longitude 
of 1 meter at the location you want.  Your code should be:
translate(rotate(scale(buffer(GeomFromText('Point(0.0 0.0)', 4326), 
r),a,b),alpha),cx,cy)

Where r is the radius in decimal degrees of 1 meter at the equator, b =1 
(the 1 meter distance does not change in the N/S direction), alpha is 0 
since you will not rotate the ellipse, and cx and cy are you final 
lat/long locations.  The a parameter is tricky.  You need to know how 
much larger 1 degree of longitude is than 1 degree of latitude is at 
cx,cy.  You might try:
 distance_sphere(GeomFromText('Point(cx cy)', 
4326),GeomFromText('Point(cx+0.01 cy)', 4326),) / 
distance_sphere(GeomFromText('Point(cx cy)', 4326) 
,GeomFromText('Point(cx cy+0.01)', 4326))
for the a parameter.

Bruce Rindahl


 David W Talmage - CONTRACTOR wrote:
> Would someone please explain the units in this snippet modeled after 
> the exampled offered in
> http://postgis.refractions.net/pipermail/postgis-users/2007-March/014971.html 
> ?
>
> translate(rotate(scale(buffer(GeomFromText('Point(0.0 0.0)', 4326), 
> r),a,b),alpha),cx,cy)
>
> What are the units of r, a, and b?  I think they are radians.
>
> My intention is to create a 1-meter radius circle at the Equator, 
> morph  it into an ellipse and then move the ellipse to an area of 
> interest on the globe to select the things that are in that area.  All 
> of my things are stored with coordinates in WGS84.
>
> I know the values of a and b in meters.  I have a function that 
> computes the number of meters per degree on an ellipsoid at any 
> latitude, so I can convert a and b to radians.
>
> I'm pretty sure that cx and cy are the longitude and latitude, 
> respectively, in degrees of the center of the ellipse.
>
> alpha is in radians.
>
> I've looked all over for an answer to this question and either I can't 
> find it or don't understand what I've found. :-(
>
>
>
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