Envi tutorial on satellite photogrammetry lab
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The document defines terms related to photogrammetry and drone surveying such as raw data, georeferencing, digital elevation models, and orthophotos. It then describes loading image data, ground control points (GCPs), and a digital elevation model into ENVI software to perform an RPC orthorectification workflow. This involves adjusting the RPC model using GCPs, checking for errors, fixing inaccuracies, and applying a geoid correction before previewing and exporting the orthorectified image.
Envi tutorial on satellite photogrammetry lab
- 1. Miloš Stojiljković Said Turksever Photogrammetry and drone surveying
- 2. Definition of terms Raw data refers to any data object that hasn't undergone thorough processing, either manually or through automated computer software. Georeferencing means to associate something with locations in physical space. Digital Elevation Model (DEM) is a specialized database that represents the relief of a surface between points of known elevation. An orthophoto is an aerial (satellite, plane, drone…) photograph or image geometrically corrected ("orthorectified") such that the scale is uniform. Orthorectification is the process of stretching the image to match the spatial accuracy of a map by considering location, elevation, and sensor information.
- 3. RPC Model Mathematical model that maps the physical relationship between 3D ground points and 2D image points. The RPC model has the advantages in being able to substitute for all sensor models, such as the projective, the linear push-broom and the SAR.
- 4. Data used 000000004627_01_P001 folder 02JAN05175446-M1BS-000000004627_01_P001 - GeoTIFF file, 2.44 m spatial resolution Associated files and data DEM folder DEM – TIFF file with 1/3 arc second resolution (10 x 10 m) GCPs folder El_paso_gcps – GCP shape file with 21 GCPs
- 5. ENVI Software – loading data
- 6. From rpc_ortho folder load raster and DEM file
- 8. RPC Orthorectification Workflow Geometric correction/ Orthorectification/ RPC Orthorectification Workflow
- 9. Adding GCPs From Load GCPs button import the shape file el_paso_gcps.shp The green check marks indicate that GCPs are active and will be used to adjust the RPC model. These points are called adjustment GCPs. GCPs properties Horizontal Accuracy value is the horizontal RMSE in meters. All of the adjustment GCPs taken together would yield an orthorectified image with accuracy to within 33 meters. Try to reduce this value by assessing the quality of your GCPs .
- 10. Errors Show error overlap button shows a color gradient of interpolated GCP errors . The white area represents a potentially large error with a GCP location. GCP 14 has an orange marker. This means that the difference between the image- estimated ground coordinate and the GCP location is greater than three times the root mean square error in the X or Y direction. Error magnitude is the length of the vector. Show error vector for viewing individual GCP errors. Delete GCP for single or all GCP
- 11. Fixing errors The cyan-colored vector shows the direction and the magnitude of the error in GCP 14 . GCP Properties shows the same vector Error X: -20.98 meters Error Y: -135.59 meters The red circle shows the circular standard error at the 95% confidence level (CE95) Errors can be fixed by changing Map X and Map Y values or by click-n-drag the marker of the point. If the fixed point visually doesn’t correspond to the GCP marker, it is better to delete it. X Y
- 12. Geoid correction DEM is required for input in order to improve the accuracy of orthorectification. The RPC Orthorectification workflow uses elevation data in conjunction with horizontal geolocation data to compute an RPC solution. The geoid correction is performed by using the Earth Gravitational Model (EGM) 1996 to automatically determine the geoid offset, displayed next to the Geoid Correction option.
- 13. The RMSE X and Y show the horizontal root mean square error, RMSE R the radius and CE95 (circular error) the circle at 95 % confidence level. The RMSE Z value is the root mean square of the Error Z values for the entire image. It reports the vertical accuracy of the entire RPC model. The LE95 (linear error) value is the difference (in meters) between the GCP-measured elevation and the DEM elevation, using the calculated geoid offset at the 95% confidence level. Accuracy
- 14. Previewing orthophoto In the GCPs tab, click the Hide error overlay and Hide error vector buttons. Set the Zoom To 200% (2:1). In the Go To field in the main toolbar, type pixel coordinates 2925, 2500 and press Enter. Enable the Preview option in the RPC Refinement panel. A Preview Portal appears, showing the orthorectification results. You should see a significant shift in the location of roads, compared to the original image.
- 15. Exporting ENVI or TIFF Choose the output file name. Enable Orthorectification Report and choose the directory for the output report. This report will list the source files used, adjustment and independent GCPs, geoid offset, error statistics, and output settings.