GEOG 372: Introduction to Remote Sensing
Fall 2007
October 17th and 18th
Laboratory 7
Geometric Registration and Mosaicking
No late labs can be accepted. Please type the answers just below the questions and hand the printouts at the beginning of the next lab: October 24th (0102) and 25th (0101). Also do not forget to mention your MAJOR on your labs
This laboratory exercise will focus on georeferencing two images to each other and stitching them together to produce a mosaic. Each of the original images covers a fairly small area. In order to produce a single image covering a larger area yet retaining the same resolution, we will mosaic these two smaller images together. Mosaicking is also used when the object of your interest is found on the boundaries of several images.
Open ENVI 4.2 and go to File - Open Image File. Navigate to U:\\g372\372-fall07\lab7data. Select TAYLO1NE.TIF. Do no assign any projection when it opens, but in the box that says Zone, enter 18. Then, just click OK in the pop-up window. Load image TAYLO1NE in Display 1 in RGB combination (Load band R in red channel, band G in green channel, band B in blue channel). Click “Load Band”. Also open TAYLO1SE.TIF using the same instructions and load it into a new display by clicking on the “Display #1” button and selecting “New display” from the drop down menu. Click “Load Band”.
Before the images can be stitched together, we first need to find the reference points where they overlap so that we know how they fit together. The first clue here is the name of the images. Each of the names contains NE or SE which stands for North East and South East respectively. Examine the images and find areas in SE and NE where they overlap.
On the ENVI Toolbar go to Map – Registration – Select GCPs: image to image. We will register the images through the selection of GCPs (Ground Control Points). In the “Image to Image Registration” pop-up window select your TAYLO1NE (whichever display you happen to have it in) as your base image and TAYLO1SE (whichever display you happen to have it in) as your warp image. Click “OK”. You will see the “Ground Control Points Selection” pop-up window.
In the “Image” display window, position the red box over the same geographic area in both NE and SE images. In the “Zoom” display window for both click on the “+” sign in the lower left corner of the display to zoom in closer to see individual pixels better. Select a pixel in the “Zoom” display of the NE (base) image and a corresponding pixel in the “Zoom” display of the SE image. Try to get as close as possible within the same pixel. It helps to position your cursor on the boundary of four pixels to avoid deviation. Once you are satisfied with the position of your cursor, click the “Add point” button in the “Ground Control Points Selection” window.
Take a screen shot of your first GCP (make sure that both zoom displays are clearly visible).
Continue the process to select 12-15 points randomly distributed in the overall area of the two images. The “Ground Control Points Selection” window will show you how many points you have selected so far under “Number of selected points”. It is better to reference the images using man-made features because they experience less change between different times than vegetation, water, or other objects. There are plenty of man-made features in these images.
When you finish selecting points, check your RMS error in the “Ground Control Points Selection” window.
1. What is your original RMS error?
Click on the “Show List” button in the “Ground Control Points Selection” window to examine the precision of your registration. Examine the RMS column of the GCP List and answer the questions:
2. What is the largest RMS error you have on your list?
3. What is the lowest RMS error?
The RMS error says how close your registration is to reality. In the real world, you should always aim for an RMS error below 0.5. Delete the point with the highest RMS error (it can be either positive or negative) from your list. See how your overall RMS error has changed and examine the list again. You will notice that the RMS error values have changed for all points once you delete one of them. Delete the point that now has the highest RMS error, and then delete the point with the next highest RMS error (make sure that you examine the list after every deletion to find the new “highest” RMS value, it will change every time!) You should have deleted a total of 3 points.
- What is your overall RMS error now?
Take a screenshot of your points list window
Select some more points if your overall RMS error is too large. In the “Ground Control Points Selection” window go to Options – Warp file, and select the SE image in the list of files in the
”Select file” window, click “OK”. In the “Registration parameters” window make sure that the warp method is RST and resampling is Cubic Convolution. Click on the memory radio dial button, and click OK. Notice that the referenced image named Memory1 is automatically added to your “Available bands” list at the top.
On the ENVI Toolbar go to Basic Tools – Mosaicking - Georeferenced. In the “Map Based Mosaic” window click on Import – Import files. In the “Mosaic Input Files” window select the NE original file and your referenced file named Memory1 (use “ctrl” to select multiple files). Leave spatial subset as “full scene” and the spectral subset as “3 bands”. You will see your images mosaicked in a new window.
Open the “Mosaic” window to the full extent and take a screen shot of your mosaic.
Copy your screenshots to a CD or USB jump drive, email them to yourself, or print them using your print account.
Log off your computer and turn off the monitor when you are finished.
Typed answers with screenshots are due October 24th (0102) and 25th (0101), 2007