Coyote Graphics Map Projection Routines

Date: Fri Nov 29 13:44:27 2013

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top cgChangeMapProjection

Map Projections, Utilities

result = cgChangeMapProjection(image, mapIn [, BOUNDARY=dblarr] [, /BILINEAR] [, LATLONBOX=fltarr] [, MAPOUT=object] [, MASK=MASK] [, MISSING=integer] [, XYRANGE=dblarr])

This function warps a map projected image from one map projection to another, using Map_Proj_Image to do the warping. Useful in general, it is used specifically to warp map projected images into the Equirectangular map projection with a WGS 84 ellipsoid that is used by Google Earth. See the program cgImage2KML to see how it is used.

Return value

An image warped into the output map projection is returned.


image in required

Any 2D or true-color image with or without an alpha channel. This image will be warped to the output map projection (mapOut).

mapIn in required type=object

A map coordinate object (cgMap) describing the map projection and coordinates of the input image that is to be warped.


BOUNDARY in optional type=dblarr

A four-element array specifying the Cartesian (XY) coordinates of the input image range, in the form [xmin, ymin, xmax, ymax]. If this parameter is not present, the boundary will be obtained from the MapIn map coordinate object. On output, this variable will contain the Cartesian boundary of the warped image.

BILINEAR in optional type=boolean default=0

Set this keyword to warp the image with bilinear interpolation. The default is to do nearest neighbor interpolation.

LATLONBOX out optional type=fltarr

A four-element float array containing the boundaries of the warped image in the [north, south, east, west] form preferred by Google Earth. Listed as degrees.

MAPOUT in optional type=object

A map coordinate object (cgMap) describing the map projection and coordinates of the warped image. This image will be warped into this map projection. The [XY]Range of this object will be set to the XYRange of the output image.

MASK out optional

Set this keyword equal to a named variable that will contain a byte array of the same dimensions as the output image, containing a mask of the “good” values. Values in the output image that were set to Missing (that is, the values were off the map) will have a mask value of 0, while all other mask values will be 1.

MISSING in optional type=integer default=0

Set this keyword equal to an integer value to be used for pixels that fall outside of the valid map coordinates. The default value is 0.

XYRANGE out optional type=dblarr

The Cartesian (XY) coordinates associated with the output image. These are the map image boundaries of the output image.


To prepare a GeoTiff file for creating a KML overlay on Google Earth:

netObject = Obj_New('IDLnetURL') url = '' returnName = netObject -> Get(URL=url, FILENAME=AF03sep15b.n16-VIg.tif') Obj_Destroy, netObject map = cgGeoMap('AF03sep15b.n16-VIg.tif') googleMap = Obj_New('cgMap', 'Equirectangular', Ellipsoid='WGS 84') warpedImage = cgChangeMapProjection(image, map, MAPOUT=googleMap)

Author information



David W. Fanning 1645 Sheely Drive Fort Collins, CO 80526 USA Phone: 970-221-0438 E-mail: Coyote's Guide to IDL Programming:


Copyright (c) 2012, Fanning Software Consulting, Inc.


Change History:

Written, 30 October 2012 by David W. Fanning. Fixed a problem with a TRANSPOSE command for true-color images. Bad logic. 4 Jan 2012. DWF. Fixed a problem in which Map_Proj_Image was not handling true-color image warping correctly and also a problem with handling missing values passed from cgImage2KML correctly. 20 Feb 2013. DWF.

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