Orthorectify satellite images with ENVI

Image providers such as DigitalGlobe strongly recommend orthorectifying satellite images to guarantee geo-accuracy. Orthorectification is the process of correcting an acquired image using an elevation model so that georeferenced pixels in the image roughly correspond to what one would see if one were flying directly over that geolocation. In this tutorial, you will learn how to orthorectify an image using a geospatial analysis software package called ENVI.

Requirements

• ENVI

Walkthrough

Locate satellite images

First locate your images. Below are the typical contents of satellite data acquired from the QuickBird satellite.

005613412020_01_P001_MUL/
    07DEC08105546-M2AS-005613412020_01_P001-BROWSE.JPG
    07DEC08105546-M2AS-005613412020_01_P001.IMD
    07DEC08105546-M2AS-005613412020_01_P001.RPB
    07DEC08105546-M2AS-005613412020_01_P001.TIL
    07DEC08105546-M2AS-005613412020_01_P001.XML
    07DEC08105546-M2AS-005613412020_01_P001_README.TXT
    07DEC08105546-M2AS_R1C1-005613412020_01_P001.TIF
    07DEC08105546-M2AS_R1C1-005613412020_01_P001.aux
    07DEC08105546-M2AS_R1C2-005613412020_01_P001.TIF
    07DEC08105546-M2AS_R2C1-005613412020_01_P001.TIF
    07DEC08105546-M2AS_R2C1-005613412020_01_P001.aux
    07DEC08105546-M2AS_R2C2-005613412020_01_P001.TIF
    EDUCATIONAL.TXT
005613412020_01_P001_PAN/
    07DEC08105546-P2AS-005613412020_01_P001-BROWSE.JPG
    07DEC08105546-P2AS-005613412020_01_P001-BROWSE.JPG.xml
    07DEC08105546-P2AS-005613412020_01_P001.IMD
    07DEC08105546-P2AS-005613412020_01_P001.RPB
    07DEC08105546-P2AS-005613412020_01_P001.TIL
    07DEC08105546-P2AS-005613412020_01_P001.XML
    07DEC08105546-P2AS-005613412020_01_P001_README.TXT
    07DEC08105546-P2AS_R1C1-005613412020_01_P001.TIF
    07DEC08105546-P2AS_R1C1-005613412020_01_P001.TIF.xml
    07DEC08105546-P2AS_R1C1-005613412020_01_P001.aux
    07DEC08105546-P2AS_R1C2-005613412020_01_P001.TIF
    07DEC08105546-P2AS_R1C2-005613412020_01_P001.aux
    07DEC08105546-P2AS_R2C1-005613412020_01_P001.TIF
    07DEC08105546-P2AS_R2C1-005613412020_01_P001.aux
    07DEC08105546-P2AS_R2C2-005613412020_01_P001.TIF
    07DEC08105546-P2AS_R2C2-005613412020_01_P001.aux
    EDUCATIONAL.TXT
GIS_FILES/
    005613412020_01_ORDER_SHAPE.dbf
    005613412020_01_ORDER_SHAPE.shp
    005613412020_01_ORDER_SHAPE.shp.xml
    005613412020_01_ORDER_SHAPE.shx
    005613412020_01_PRODUCT_SHAPE.dbf
    005613412020_01_PRODUCT_SHAPE.shp
    005613412020_01_PRODUCT_SHAPE.shx
    005613412020_01_STRIP_SHAPE.dbf
    005613412020_01_STRIP_SHAPE.shp
    005613412020_01_STRIP_SHAPE.shx
    005613412020_01_TILE_SHAPE.dbf
    005613412020_01_TILE_SHAPE.shp
    005613412020_01_TILE_SHAPE.shx
005613412020_01_LAYOUT.JPG
005613412020_01_LAYOUT.JPG.aux.xml
005613412020_01_LAYOUT.JPG.xml
005613412020_01_LAYOUT.aux
005613412020_01_LAYOUT.rrd
005613412020_01_README.TXT
005613412020_01_README.XML
schema.ini

The 005613412020_01_P001_MUL folder contains the multispectral images and the 005613412020_01_P001_PAN folder contains the corresponding panchromatic images. Here we have rearranged the files into pairs.

07DEC08105546-M2AS_R1C1-005613412020_01_P001.TIF
07DEC08105546-P2AS_R1C1-005613412020_01_P001.TIF

07DEC08105546-M2AS_R1C2-005613412020_01_P001.TIF
07DEC08105546-P2AS_R1C2-005613412020_01_P001.TIF

07DEC08105546-M2AS_R2C1-005613412020_01_P001.TIF
07DEC08105546-P2AS_R2C1-005613412020_01_P001.TIF

07DEC08105546-M2AS_R2C2-005613412020_01_P001.TIF
07DEC08105546-P2AS_R2C2-005613412020_01_P001.TIF

Locate digital elevation model

Orthorectification requires a digital elevation model, which are measurements of elevation at each geolocation. Results from the latest Shuttle Radar Topography Mission are provided free at the CGIAR-CSI SRTM 90m DEM Digital Elevation Database.

Navigate to the SRTM search page.

Download your localized SRTM data as a GeoTiff image.

Perform orthorectification

Start ENVI and choose Map > Orthorectification > QuickBird > Orthorectify QuickBird. Open the satellite image you want to orthorectify and click OK.

The Orthorectification Parameters dialog appears.

The ENVI documentation recommends using Nearest Neighbor resampling to preserve pixel values.

• Set Image Resampling to Nearest Neighbor
• Set DEM Resampling to Nearest Neighbor

Select elevation model.

• Click on Select DEM File
• Open the SRTM GeoTiff image
• Select Band 1
• Click OK

Begin orthorectification

• Choose an output file path
• Click OK

The orthorectification process can take anywhere from ten minutes to an hour, depending on the size of the image.