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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