[Proj] Proj4 Projection Coordinates for Geostationarysatellitedata

Tue Aug 13 10:33:27 PDT 2013

Kathryn,

Just thinking now … if you've got a vertical perspective (find the formulas for vertical perspectives in section 1.3.17.2 of Guidance Note 7 Part 2 at www.epsg.org) on a spherical earth, the horizon is a fixed number of grid units from the center, i.e. the lat/lon of your geostationary satellite on the Earth, which is (0,0) in grid units.  The formulas in GN7-2 will help you compute that distance, but no matter.  Just experiment until you find the horizon by trial and error.  That is, if (Northing^2 + Easting^2) > (fixed distance)^2, blank that point out.  Now, this simplicity would not be possible with an ellipsoidal Earth ...

Noel

Noel Zinn, Principal, Hydrometronics LLC
+1-832-539-1472 (office), +1-281-221-0051 (cell)
noel.zinn at hydrometronics.com (email)
http://www.hydrometronics.com (website)

From: Kathryn Jablonski - NOAA Affiliate 
Sent: Tuesday, August 13, 2013 11:00 AM
To: PROJ.4 and general Projections Discussions 
Subject: Re: [Proj] Proj4 Projection Coordinates for Geostationarysatellitedata

Hi Noel,  

We set up the above definition from following the proj4 definition of geostationary.  We have also tried this string: 
Proj("+proj=geos +h=35786023 +a= 6378137.0 +b= 6356752.3141403561 +sweep=y +lon_0=-75 x_0=-.151844 y_0=.151844 +units=meters +no_defs") and still get the same error for the off earth data.  

I get these as the output extents:[1e+30, 1e+30, 1e+30, 1e+30] and the error given is this: RuntimeError: tolerance condition error

On Tue, Aug 13, 2013 at 10:43 AM, Noel Zinn (cc) <ndzinn at comcast.net> wrote:

  Your graphic shows the earth profile as an ellipse, but your proj4 string sets a and b to the semi-major axis of WGS84 (i.e. a sphere).  Why?  -Noel

  Noel Zinn, Principal, Hydrometronics LLC
  +1-832-539-1472 (office), +1-281-221-0051 (cell)
  noel.zinn at hydrometronics.com (email)
  http://www.hydrometronics.com (website)

  From: Kathryn Jablonski - NOAA Affiliate 
  Sent: Tuesday, August 13, 2013 9:38 AM
  To: PROJ.4 and general Projections Discussions 
  Subject: Re: [Proj] Proj4 Projection Coordinates for Geostationary satellitedata

  Thanks Janne.   

  We did try drawing a limiting box inside the earth extents to get all valid data points within, and this works but cuts out data in the edges of the earth.  See the following image:  

  We still need the data in between the drawn dotted line blue box and the edge of the earth.  The data values in the red shaded off earth areas are very large (2.14 E9) and these are the values that throw the error.  If you do discard the values in the light red, that is fine but it is throwing an exception instead, preventing us from proceeding forward.  

  Thanks for the support!

  On Tue, Aug 13, 2013 at 3:12 AM, <support.mn at elisanet.fi> wrote:

    Hello,

    we just discarded all points outside any reasonable area.
    Another possibility might be to draw a limiting line (or box)
    and move all outside points to that..

    Hope that helps?

    Regards: Janne.

    ----------------------------------------------------------------------------------------

    Kathryn Jablonski - NOAA Affiliate [kathryn.jablonski at noaa.gov] kirjoitti:

    > I am trying to convert geographic coordinates (in degrees east/north lat
    > lon) to projected coordinates in meters for geostationary full disc data
    > (GVAR Goes East data from CLASS).  To do this, I tried using PyProj/ Proj4
    > and am running into errors due to the off earth pixels in the corners.
    >
    > This is the proj4 string given:
    >  projection_coords = Proj("+proj=geos +h=35774290 +a= 6378137 +b= 6378137
    > +lon_0=-75 +units=meters +no_defs")
    > ll_x, ll_y = projection_coords_geos( LL_y_deg, LL_x_deg, inverse = False,
    > errcheck=True)
    >     'x=%9.3f y=%11.3f' % (ll_x,ll_y)
    >     ur_x,ur_y = projection_coords_geos( UR_y_deg, UR_x_deg, inverse =
    > False, errcheck=True)
    >  'x=%9.3f y=%11.3f' % (ur_x,ur_y)
    >
    > For full disc geos data I am receiving incorrect (1e30) values for the
    > extents and was wondering if anyone has ever run into the same error or
    > have a suggestion to correct this.
    >
    > Also, for remapping GOES data, should the 'sweep axis' be including in the
    > Proj string and set to 'y'?
    >
    > Thanks in advance for any suggestions!
    >

    > _______________________________________________
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    > Proj at lists.maptools.org
    > http://lists.maptools.org/mailman/listinfo/proj

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