Terrain Variation

5.3.2.1 Elevation

Elevation was plotted against reflectance values for a number of vegetation class pairs to determine if it could be used as a separation feature. Figure 5.14 illus-trates one example of these. The vegetation classes 7c (Enggranskog VG) and 9c (Grasmyr LG) show that it is fairly possible to separate between these two types in the NIR band using elevation, even though they both cover similar reflectance

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values. Elevation is a good attribute to add for separating vegetation types occur-ring in the valley with those higher up on the slopes, but that both cover similar reflectance values. 7c and 9c had a JM distance of 1.94 after buffering. Of the 10 plots for elevation using the selection of vegetation types in figure 5.8, only 2 were separable. The second pair was 7b and 2e, but most of the polygons in these classes belong to the same grazing quality class.

Figure 5.14: Scatter plot, elevation of 2 vegetation classes against the NIR band. Image 24.07.94

5.3.2.2 Slope & Aspect

The affects of slope and aspect on different vegetation types was illustrated through polar plots. Each dot was placed in a position on the polar plot depending on its slope and aspect value. A colour code represents the reflectance value range. The distance from the centre represents the slope of the pixel. Slope was recorded in whole integer numbers, which gives rise to the ring appearance of the plot (i.e.

each ring representing a degree of slope). The further out the data point is lying, the higher slope value. The angle measured from the centre represents the aspect.

North is to the right and continues in a clockwise direction. There are 6 plots one for each Landsat TM band of data.

Figure 5.15 shows the data for vegetation type 7b (spruce with blueberry under-growth). This type covers 13% of the Venabygd site and is categorised as having potentially good grazing quality. Those data points having a slope of approx. 6^◦ or less appear to lie in a complete circle around the centre. This means that pixels with a low slope angle show no directional preference (i.e. specific aspect). This seems intuitive, as relatively flat ground would receive approximately the same amount of sun and water. When the slope angle increases however, the way in which the slope is facing becomes more important. Data points with slope values greater than6^◦ show more of a directional preference. From the plots of vegeta-tion type 7b it appears that most of the vegetavegeta-tion lies in areas that have aspects ranging between north and southwest. The majourity of the data was lying on slopes that are≈20^◦ or less.

Each plot represents a distinct reflectance value category. Band number 1, the blue band, has reflectance values that range from 5-17%, so it has a navy blue colour with a few orange spotse. Band numbers 2, 3 and 7 are red and hence have reflectance values from 0-5%, but there are also several blue and orange spots, indicating that some points have reflectance values up to 15%. Band 4 on the other hand is mostly green, in the 16-20% range. When looking closely there are also circles of red, blue, orange, gold, sienna and pink. In this band, for vegetation type 7b, the reflectance values range from 1-37%.

Similar plots were drawn for vegetation types 7b, 4b, 2e, 2c, 9c, 9a, 11b and 7c. Vegetation types 2e, 2c, 9c, and 9a, showed a fairly circular pattern meaning that as the slope increased they did not show any signs of being distributed on a particular slope direction (i.e. aspect). These vegetation types are alpine grasses, heath and swamp vegetation. 11b, 4b, and 7c however, showed more direction choice. 11b is pastures, and had a south, south-west direction on all slope angles although the slope angle range only went up to ≈ 10^◦. 7c, another spruce type showed fairly similar results to the 7b (spruce) plots (fig 5.15), with the vegetation lying mostly on slopes that faced in a west, south-west direction. The notable point in this plot was that there were almost no points towards the middle of the graph indicating that this vegetation type occurred at angles greater than 5^◦ (but less than 10-12). 4b, a birch type, covered a large range of aspect angles especially in very low slope angles. Above≈ 9^◦ most of the points were found in a south-west through to northerly direction. Each vegetation type had unique reflectance colour combinations for the different bands.

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Reflectance Category colour

0-5% Red

6-10% Navy Blue

11-15% Dark Orange

16-20% Spring Green

21-25% Gold

26-30% Sienna

31-35% Deep pink

Figure 5.15: Vegetation type 7b. Band: 1, 2, 3, 4, 5, & 7, from left to right. Slope is given as the distance from the middle. Aspect as the angle from the centre, see north diagram. The reflectance value is represented in a series of colours. The size of the data point represents the frequency of the data value combination.

5.3.2.3 Atmospheric Variation

The electromagnetic radiation signals collected by satellites in the visual spec-trum are affected by scattering and absorption by gases and aerosols as they pass through the atmosphere. When and how to correct the atmospheric effects depend on the remote sensing and atmospheric data available, the information desired, and the analytical methods used to extract the information. In many applications involving classification and change detection, atmospheric correction is unnec-essary, as long as the training data and the data to be classified are in the same relative radiometric scale. Corrections are necessary however if multi-temporal datasets are to be used (Song et al., 2001). Due to the fact that an image from only one time period has been used in most analyses, no atmospheric corrections were performed. Cingolani et al. (2004) also agrees that it was unnecessary with atmospheric correction when a single image is to be used for classification.