GEOM30009 IMAGING THE ENVIRONMENT

Assignment 4
Image Georeferencing and Orthorectification


Due for submission at 11:55 pm on Friday of Week 12


Objective
Value: 15% of Subject Mark
The aim of this assignment is to learn how to georeference a satellite image, generate a Digital
Elevation Model (DEM) from a pair of images and use the DEM to orthorectify an image. You will
use a stereo pair of Pleiades imagery taken from Melbourne, Victoria. You will learn how to rectify
the images using a set of Ground Control Points (GCPs). Further, you will generate a DEM from the
stereo imagery. Then, you will orthorectify the images and assess the correctness of
orthorectification by overlaying a vector map of the region on the generated orthophoto.

Background
Georeferencing and orthorectification are critical steps to enable geometric measurements on
remotely-sensed images. Georeferencing refers to the relationship between the image coordinate
system and a geographic coordinate system. The pixels in a non-georeferenced image do not have
ground coordinates (e.g., X, Y, and Z) which are fundamental for measurements such as distance
and area. During the georeferencing process, the effect of image perspective (tilt) will be removed.
On the other hand, orthorectification is the process of removing not only tilt distortion but also
relief displacement. This enables overlaying existing vector maps on the image with good accuracy.
Given the image coordinate and the geographic coordinate of a set of GCPs, e.g. road junctions or
building corners, the georeferencing can be done by estimating a transformation between the
image coordinate system and the ground coordinate system. Orthorectification is performed by
using a DEM and a set of GCPs. The georeferenced and orthorectified image can be used in many
applications when metric information is crucial.

Data
A stereo pair of Pleiades satellite imagery, along with a parcel map of Melbourne will be used for
this assignment. The dataset can be downloaded from LMS under “Assignment 4: Image
georeferencing and orthorectification”.

Software
ENVI will be used for reading the dataset and GCPs, estimating the transformation and generating
a georeferenced image. To generate a DEM, a pair of images with sufficient overlap is used.
Following the DEM generation, an orthophoto will be generated for either of stereo images.

Tasks
The assignment consists of three tasks:
1. Georeferencing a satellite image and its quantitative accuracy assessment.
2. Generating a DEM using a stereo pair of imagery.
3. Generating an ortho image and verifying the orthorectified image using a fine-scale map.
The whole assignment should be completed within three weeks.

Task 1: Georeferencing a satellite image
In the task, you will georeference a given satellite image using a number of GCPs with known ground
coordinates provided in a pts file.

Steps:
1. Copy the required files from the LMS to your local disc area (Desktop in FEIT GPU).
2. Unzip both Pleiades datasets.
3. Start ENVI and open the Pleiades image set: File > Open As > Optical Sensors > Airbus >
Pleiades > DIMAP(.DIM)
4. In Pleiades window, for each image, open the “IMG_PHR1A_P_001” folder and choose the
DIM_PHR1A_P_... XML file. Repeat this step for both images (Folders 6921741101 and
6921742101).
5. From the toolbox (on the right side of the software), click Geometric Correction and then click
Registration and select Registration: Image to Map.
6. For each image, choose the R (i.e., panchromatic), G (i.e., panchromatic) and B (i.e.,
panchromatic) bands according to RGB layers of the image and press OK.
7. In the Image to Map Registration window, set the projection parameters as UTM (project
system), WGS-84 (Datum), Meters (unit), 55 (Zone), S (South), 0.6 meter pixel size and press
OK.
8. From the menu in the Ground Control Point Selection window, select File > Restore GCPs
from ASCII and choose the corresponding GCP file *.pts for the selected image. For example,
if you work on 6921741101 image, you should select the GCP file GCP_6921741101.pts.
9. Having selected the GCPs, the software allows you to see the calculated RMS error for all
GCPs in the lower left corner of the Ground Control Point Selection window. However, this
value is beyond the acceptable range, meaning the transformation model is not
accurately estimated.
10. To accurately estimate the transformation model, the polynomial degree should be changed
to 2. To do this, you can simply increase the degree in the Ground Control Point Selection
window. In order to evaluate the georeferencing process, we need to have some test points.
For doing that, you need to exclude at least 5 GCPs from the process. By clicking on Show
List, deactivate those GCPs which have high RMSE values. This can be done by simply clicking
on on/off icon in Image to Map GCP List. Please note that an RMSE of 3 pixels or smaller is
considered acceptable for this task, so pay attention to exclude proper GCPs. To easier
exclude GCPs with large error, you can go to Options and sort the GCPs according to their
error.
11. Once the georeferencing parameters are estimated, in the Ground Control Point Selection
window click Options and then select Warp Displayed Band. Generate a warped image and
the image will be added to the Layer Manager window. Remember not to close the Ground
Control Point Selection window, since you will need it in the next steps.
12. Now every pixel in the image should have ground coordinates. To see the ground
coordinates of a pixel, you can click Cursor Value icon indicated by or simply press Ctrl + I.
13. To assess the accuracy of the georeferenced image, choose the excluded ground control
points that have not been employed in the georeferencing process and are distributed across
the image. Determine their ground coordinate (Cursor value) from the georeferenced image,
then compute the errors (Equations provided below). These points are check points since
they have no role in georeferencing estimation. In order to find the location of each check
point on the georeferenced image precisely, first zoom in to the image (e.g., to 60%) and find
its map coordinate (i.e., Map X, Map Y) from the list of points (in the Ground Control Point
Selection window). In Go To box (next to drop down list Annotation on the tab bar), insert
the Map coordinate (Map X, Map Y) to moves to the view screen, which has the proximity of
the wanted point. From here, navigate to the corresponding point of the check point on the
georeferenced image to measure its coordinates with the cursor. You can find a guide
describing locations (GCP_Locations.docx) of points on the image, together with their
ground coordinates in the GCP_6921741101.pts or GCP_6921742101.pts files.

Note: In your report, you should discuss how the generated georeferenced image is different from the
original image, and calculate the RMSE over the check points by computing the difference between
their real ground coordinate (in the GCP_6921741101.pts or GCP_6921742101.pts) and the
corresponding ground coordinates (cursor values) measured on the georeferenced image.

To calculate the Root Mean Square Error (RMSE), first calculate the error of X and Y coordinates for each
point i using the following equation:



(1)

Consequently, the mean error in X and Y (MEX and MEY) is using the following equations:



(2)

Finally, calculate the Root Mean Square Error in X and Y (RMSEX and RMSEY):



(3)

and the Combined RMSE (RMSEP):

→

(4)
In the above equations, the superscript chk refers to coordinates of check points determined from
the georeferenced image (cursor value) and ref indicates the real ground coordinates of checkpoint
from the given text file. You can save the corresponding values in an Excel file for further calculation.

Steps for error calculation using the 10 check points selected in the previous step:
1. Identify and fill into an Excel sheet the X and Y coordinates of the provided check points in the
georeferenced image (Xchk, Ychk) through right clicking on the layer and choosing Cell
Coordinate.
2. Calculate error in X and Y per point (i.e., and ) using equation (1).
3. Calculate the Mean Error in X and Y (i.e., MEX and MEY) using equation (2).
4. Calculate Root Mean Square Error in X and Y (i.e., RMSEX and RMSEY) using equation (3).
5. Finally, calculate the combined RMSE (i.e., RMSEP) using equation (4).
6. Save the Excel file that includes the derived X, Y coordinates and the results of calculations.

Note: In your report, you should discuss the factors affecting the RMSE of the georeferencing
process.

Task 2: Generating DEM from a stereo imagery
We will generate a DEM of the region which will be used for orthorectification.

Steps:
1. Open ENVI.
2. Same as Task 1, open the Pleiades image set: File > Open As > Optical Sensors > Airbus >
Pleiades > DIMAP(.DIM). In Pleiades window, for each image, open the “IMG_PHR1A_P_001” folder
and choose the DIM_PHR1A_P_... XML file. Repeat this step for both images (Folders 6921741101
and 6921742101).
3. From toolbox, find Terrain- DEM Extraction and double click on it.
4. Select DEM Extraction Wizard: New.
5. In the opened window click Select Stereo Images, then navigate through the image files
and select the images. Make sure to select the image in “6921741101” folder
(IMG_PHR1A_P_202312200026154_SEN_6921741101-1_R1C1.tif) as left and image in
“6921742101” (IMG_PHR1A_P_202312200027190_SEN_6921742101-1_R1C1.tif) as right
images. Click OK and go to the next step by clicking on Next in DEM Extraction Wizard
window.
6. Select radio button Read GCPs From File and by pressing Select New GCP File select
GCPs_Melb.pts provided and set the projection parameters as UTM (project system), WGS-84
(Datum), Meters (unit), 55 (Zone), S (South). Then, by pressing Next proceed to the next step.
7. Choose Read Tie Points From File, then select the TiePoints.pts.
8. Change the directory of outputs as you wish otherwise leave it unchanged and go to the next
step.
9. Be sure that parameters are correct, especially x and y pixel size must be set as 0.6 meter.
Then, press Next.
10. Choose the directory for storing the DEM file.
11. By clicking Load DEM Result To Display, you enable to see the result of generated DEM and
visually evaluate it. Click finish and do not save the current state of the wizard.
12. In order to see the generated model in true visualization, first you should close all the
DEM windows, then from the Toolbox, under Terrain, click the 3D SurfaceView and in the opened
window, choose the panchromatic layers of either of the images and press OK. In the next window
select the DEM file and click OK. Now in the next window, choose the parameters that you would like the
3D model is created based on. Select DEM resolution to 512 and the image resolution to 1024. This
setting is dependent on the memory of the system you are using and in case of errors, reduce the
resolutions. Other parameters could be left as their default. By pressing OK you would see the 3D
model of the generated DEM.
13. To see the generated model in RGB visualisation, you can open Multi Spectral image of either
images (Left or Right) which are in IMG_PHR1A_MS_002 folders, then repeat previous step, by
selecting RGB layers of the images rather than their panchromatic layers.

In your report, you should include a screenshot of the generated DEM and provide a visual
interpretation of the elevations and the topography of the area (Both panchromatic and RGB
visualizations are acceptable).

Task 3: Orthorectification
Having generated a DEM, we can orthorectify the image to remove the effect of relief
displacement.

Steps:
1. Open ENVI.
2. Same Task 1, open the Pleiades image set: File > Open As > Optical Sensors > Airbus >
Pleiades > DIMAP(.DIM). In Pleiades window, for each image, open the “IMG_PHR1A_P_001”
folder and choose the DIM_PHR1A_P_... XML file. Repeat this step for both images (Folders
6921741101 and 6921742101).
3. Navigate through toolbox Geometric Correction> Orthorectification> RPC Orthorectification
Workflow.
4. In the opened window, choose your input file which can be either of stereo pair image and for
DEM file select the generated DEM file.
5. In the next step, from Load GCPs choose the either GCP_6921741101 or GCP_6921742101
file, depending on the selected image. You see that all the GCPs will be listed and there is a
green check next to each point indicating the contribution of points to georectification.
6. To evaluate the accuracy of image orthorectification, we should have some independent
points which don’t contribute to the process. To have these points, change the status of at
least 5 points from Adjustment to Independent. Try to select these points across the image so
that they cover the entire image.
7. Now by going to Statistics you enable to see horizontal and vertical accuracies for adjustment
GCPs as well as independent GCPs. Summarize the result of accuracies in a table and bring it in
your report in a very clear way.
8. Once the orthophoto has been generated, load the shape file of parcels from folder Parcel
Map, i.e. Parcels.shp. This vector map should precisely overlap with the orthophoto image.
Take a snapshot of both layer and include it in your report. You should include the entire data
as well as a few zoomed-in snap shots verifying both layers overlap properly.


Submission
Write a 2500 ± 10% word scientific report and include the following content:
1. Provide a proper introduction with background information, addressing the purpose of
georeferencing, DEM generation and orthorectification of satellite imagery.
2. In the Methods section describe the process you performed to complete each of the three
tasks. You can also include intermediate results if they help you describe the process.
3. Include the result of each process in result section. (e.g. snapshot of georeferencing result, 3D
model of the DEM, and the orthorectified image overlapped with the vector map, accuracy
results for georeferencing and orthorectification.)
4. In the Discussion section provide an analysis of your results and address the following
questions:
i. How does the distribution of GCPs impact on the accuracy of georeferencing? What is the
role of GCPs in georeferencing?
ii. As noted, the key difference between a georeferenced image and an orthorectified image
is the removal of relief displacement. Describe what this effect is and how it is removed
during the orthorectification? You can associate your answer with some illustrations.
iii. Assuming an accuracy of 15 m for the DEM and a field of view of 1.83 degree for the
Pleiades image, what is the maximum radial error of the orthorectified image?
5. Provide a clear and concise conclusion summarizing your findings.
6. Provide a reference list for your supporting research.
7. Statement of contribution – include a statement describing the contribution of each group
member to the assignment tasks and the report.

Submit a digital version of your report via LMS and in pdf format only.
Marking rubric
Appropriate length and proper formatting 5%
Proper introduction 10%
Proper method section 10%
The georeferenced image present and correct 10%
The RMSE correctly calculated and reasonable related to georeferencing 10%
The DEM image present and correct 10%
The orthophoto overlaid with vector map present and correct 10%
The RMSE correctly calculated and reasonable related to orthorectification 10%
Three questions answered and properly discussed 15%
Logical conclusions 10%


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