Cairo - Exercises using Landsat data - Continued
|  | Measuring circumferences | |
Exploring Cairo
A normal Landsat scene covers an area of approximately 180 x 180 km². The segments used for the case studies are smaller, covering 512 x 512 pixels.
The resolution of the panchromatic image is 10 metres. All other images have a resolution of 30 metres. That means one pixel shows a square with an area of 30 x 30 m².
Calculate the width of the Landsat scene segment Cairo_Landsat_2000_Band_321.tif by using this information.
Surface Calculating
The measurement of distances and circumferences is a very common feature in the use of satellite images. It is easier and cheaper than employing surveyors in the field. The selection of the appropriate band combination depends on the object of interest. Vegetation, for example, is best seen in an infrared false-colour combination, because plants reflect much more infrared radiation than visible light. On the other hand, islands are best seen in infrared images, because the surrounding water does not reflect infrared radiation and therefore appears black.
Open the LEOWorks programme. If you have not downloaded the images of Cairo yet, do so now.
Choose File>Open. A dialogue box will pop up. Choose the folder Cairo and select image Cairo_Landsat_2000_Band_4.tif.
The image needs to be improved in brightness and contrast.
Choose Enhance>Histogram Equalization. The image will change.
Choose Image>Measure tool and select Units>Metres.
You do not have to enter the pixel size since the GeoTIFF format used for the data contains this information.
Draw a polygon around the Northern island. Start south by clicking on the right mouse button. Draw the polygon along the coastline and close it by using the left mouse button.
All information is shown in the dialogue box: the distances of every single vector, the length of the polygon and the surface area.
Convert the m² into km². Compare the measured circumference with those obtained by your classmates.
Position
Every single pixel of a Landsat image is clearly defined by its 'x-y' position and the digital value. Different positions with the same digital value indicate equal objects. It does not mean that they have to be the same, but they could be. You need to classify the objects to be sure.
Open the LEOWorks programme. If you have not downloaded the images of Cairo yet, do so now.
Choose File>Open. A dialogue box will pop up. Choose the folder Cairo and select image Cairo_Landsat_2000_Band_4.tif.
The image needs to be improved in brightness and contrast.
Choose Enhance>Histogram Equalization. The image will change.
Locate the cursor position (226, 196) -> (247).
What kind of geographical object can you identify?
What kind of surface do the digital values indicate (e.g. 247)?
Move the mouse crosshair across the image to compare the different digital values.
| | | Transparent overlay combined image |
Transparent Overlay
Sometimes it is useful to highlight certain features such as water bodies. The advantages of an infrared image for the determination of water surfaces can be merged with the visualisation power of a true-colour image by using a transparent overlay.
Open the LEOWorks programme. If you have not downloaded the images of Cairo yet, do so now.
Choose File>Open. A dialogue box will pop up. Choose the folder Cairo and select the first image Cairo_Landsat_2000_Band_321.tif. Open Cairo_Landsat_2000_Band_4.tif, too.
The image Cairo_Landsat_2000_Band_4.tif needs to be improved in brightness and contrast.
Select (activate) the image Cairo_Landsat_2000_Band_4.tif and choose Enhance>Histogram Equalization. The image will change.
Open Image>Transparent Overlay and choose
Cairo_Landsat_2000_Band_4.tif as BackGround Image and
Cairo_Landsat_2000_Band_321.tif as ForeGround Image.
Now choose 50% as positioning for the Opacity-slide.
Are the previously described land use classes more visible, and if so, why?
Try other transparent overlays. For example, highlight the vegetation or the densely built-up areas in a true-colour image.
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