AERIAL PHOTOGRAPHS

'Black and white' aerial photographs are in reality grey scale images, with autoscaled values ranginging from zero (usually deep water bodies) to 255 (snow or overexposed quartzite). Consequently, the grey scale image can be displayed as a colour image simply by the formulation and application of a suitable colour palette.

[Note: a Byte binary file is a binary image file in which all cells have values within the range of 0 to 255 (positive only) and can thus be stored as single-byte integers rather than the two-byte integers normally required. Byte files take half the disk space required for normal integer files and may be created with CONVERT. In cases where the conversion involves a change from real numbers to either integer or byte, you will be offered the choice of conversion by truncation (fractional portions are chopped off) or by rounding. If the conversion results in a packed binary file (run-length encoded) and the resulting data values fall within a range of 0 - 255, the data type will be forced to byte regardless of the initial type chosen, and a message will appear. Conversion of integer files to a packed binary format does not always lead to data compaction. The packed binary format is by run-length encoding. The image is therefore stored as a series of number pairs with the first number indicating the run or number of repeats and the second number indicating the attribute code or data value. If the image contains many areas of the same value such as with most map data, then the runs will be large and the packed binary format will lead to substantial compaction. If most cells tend to have different values from their neighbors such as with satellite imagery or digital elevation models, then the runs will be small and the compaction will be minimal or absent -- indeed, the resulting file may even be larger than the original! If you are in doubt about the efficacy of using CONVERT to pack a file, try it! The procedure is fast enough to compare the compacted file size with the original, and if it doesn't help, use CONVERT to decompress it back to its original form.]

Water might be represented by a colour range from black to dark blue, snow by a range from pink to white, vegetation by a range of colours in orange and/or green, and rocks in a range of colours in cyan, magenta, yellow and red. Since the grey scale ranges for different objects will overlap, it will require some experimentation to formulate an optimized palette, and the palette will need to be customized for each photograph to be coloured. Although the certainty of identifying objects on the basis of their grey scale value is subject to the limitations imposed by overlapping values for different objects, the addition of colour can help to differentiate non-random variation within the range of grey scale values represented by the photograph. The procedure used in this exercise is similar to that used in the classification of objects in remote sensed images (LANDSAT, SPOT, RADARSAT). Such images are of low resolution (10 - 30 metres) compared to air photos, but have a much greater range of spectral information. The exercise will use a pair of stereo photographs of the 3rd year field camp area north of Coniston, Sudbury. Rock units displayed on the photographs include Mississagi Quartzite, large bodies of intrusive Nipissing gabbro, and Sudbury breccia, as well as snow, vegetation, bog and water. The images represent about one quarter the area of a standard 9 x 9 inch airphoto. The photos were digitized at 600 dpi, and each image file has a size of about 6 Megabytes. Reducing the resolution to a half (300 dpi) diminishes the file size to one quarter, i.e. digitizing a whole airphoto at 300 dpi requires about 6 Megabytes of disk space. Sixteen standard air photo images could be transported on one Iomega diskette.

Operation:

1) Map the Network Drive to your machine:

Fetch NT Explorer

Click Tools

Click Map Network Drive

Click Earthnt

Click public

Click OK

This will set a drive letter for the path Earthnt\public

2) Load Idrisi and set the environment to the 'mapped drive'\image, i.e. Earthnt\public\300b\image.

Display image 4621-12b with a 256 grey scale palette.

Using the Query tool to determine the range of grey values representing the various components of the image, e.g. snow, dark rocks, light rocks, vegetation, water, etc.

Fetch the Palette Workshop and save the palette with the name pal'your intials1'.

Colour your palette to give each component of the image a different range of colours.

Create another palette pal'your intials2' that will block colour the components of the image.

How to generate a digitized vector image.

3) Click the Help button and search for 'Digitize'. Read the instructions carefully, and then apply them to produce a digitized vector image of the Mississagi Quartzite.

Display the vector image produced (no need for explanation here, is there???).

How to generate an enlarged 3D image from a pair of digitized airphotos

4) Display the stereoscopic images 4621-12b and 4621-13a using the GARSON2 palette. Zoom the images to the area indicated by the lab instructor. Match the images to produce a stereo view of the area, and identify the folds in the Mississagi quartzite unit.

FIGURES :

Figure 1. Plate Tectonics.

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