Your parameter r is in map units. For your example r = 1 or a 1 degree
circle. The center section should be :
buffer(GeomFromText('Point(0.0 0.0)', 4326), 1)
Now you have a circle (polygon actually) with a radius of 1 unit
(degrees) centered at point 0,0.
Next you need to scale the circle to create an ellipse. You do this
with the scale factors a and b. These parameters are dimensionless
(ratios).
The query becomes scale(buffer(GeomFromText('Point(0.0 0.0)', 4326),
1),a,b) where a is the meters per degree of longitude and b is the
meters per degree of latitude at your final point cx,cy. This gives you
an ellipse at point(0 0) with the correct shape.
Next is a rotation function to create an arbitrary ellipse. You do not
need this so eliminate it.
Finally translate the ellipse to your desired location - cx cy.
translate(scale(buffer(GeomFromText('Point(0.0 0.0)', 4326), 1),a,b),cx,cy)
Read it from the inside out. Make a unit circle (buffer) then scale it
differently in x and y (scale (...),a,b) then move it to the location
you want (translate(...),cx,cy)
Bruce
David W Talmage - CONTRACTOR wrote:
Bruce Rindahl wrote:
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:
...
Thank you for helping me!
I should have worded the problem differently. I need to make an ellipse
with a and b greater than 1. Following the code, I start with a 1m
circle, then scale it up to a-meters by b-meters.
So the r, a, and b are all in degrees? Then I have to figure out how
many meters per degree at latitude cy and divide r, a, and b by that
number, right?
If that's right, then something strange is going on. I'm creating the
ellipse that way and my SELECT asks for all things whose bounding box
intersects that of the ellipse. PostGIS is returning far fewer things
that I expect. Here's my SELECT:
SELECT rf.name FROM raster_entry re, raster_file rf WHERE
re.raster_data_set_id = rf.raster_data_set_id AND rf.type = 'main' AND
(re.ground_geom &&
translate(rotate(scale(buffer(GeomFromText('Point(0.0 0.0)', 4326),
?), ?, ?), ?), ?, ?));
I'm doing this in Java with a PreparedStatement. The '?' correspond
to r, a, b, alpha, cx, and cy, respectively.
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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