Reprojecting Images

GeoExpress allows images to be reprojected from one coordinate reference system (CRS) to another. The output formats that support reprojection are MG3, MG4, JPEG 2000 (JP2) and NITF* 2.1.

Both a source and an output CRS must be specified for a reprojection operation. You may enter any CRS defined by the European Petroleum Survey Group (EPSG). Alternatively, you can enter a well-known text string (WKT) that describes the coordinate system.

NOTE: If you create a mosaic from images in different projection systems, you do not need to specify an input coordinate reference system with the -fromepsg switch. For more information on mosaics with multiple projections, see Creating a Mosaic.

The error fraction is a value from 0.0 to 1.0 that indicates the level of accuracy of the reprojection. A value of 0.0 is the highest level of accuracy – basically a "lossless" reprojection. An error fraction of 0.125 means that each reprojected pixel could be off by an eighth of a pixel, while a value of 0.25 indicates the allowance of a quarter pixel inaccuracy.

Of the resampling methods, nearest neighbor is the fastest method, while bicubic gives more accurate results. Bilinear offers a compromise, faster than bicubic but more accurate than nearest neighbor.

Use the -reprores flag to change the x and y output resolution of the reprojected image. If you manually set the resolution and you notice that the image is offset, you may want to use the -alignpixels switch. For more information, see Encoding Options.

NOTE: Reprojection is an encoding operation, meaning that compressed file formats such as MrSID and JPEG 2000 will be decompressed and re-encoded during the reprojection operation, which could result in a loss of image quality.

*Support for encoding NITF images is available for GeoExpress. Contact your LizardTech representative to purchase the NITF Extension for GeoExpress.

Example

mrsidgeoencoder -i foo.tif -o foo.sid -fromepsg 4326 -toepsg 26718

Reprojection Switches