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Global Environment
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Land Cover Map of West Asia Using 1-Km AVHRR Data
Pre-Classification
In order to study the realationshp between the different values of NDVI through the four season of the year, four histograms of NDVI monthly composites data representing the four seasons were created, other four histograms were created using the vegetation cover mask. We conclude that January has lowest amount of vegetation. April period has an important vegetation activities in the north of the study area, while high vegetation activities appears in southern part during the period of October, which is justified by the sub-tropical climate, the period of July has a medium vegetation activities especially in southern and northern parts, and few or non vegetation activities in middle part of the study area.
Knowing that the study area cover a relatively large surface 4200 km X 3600km, there is insufficient knowledge exist about its, and spectral signatures show high variability, so the unsuperivised classification or clustering is a very useful tool in this case. To achieve this classification, we used the K-Means algorithm. We obtained about 15 clusters which should be interpreted in terms of land cover classes. For his step of inerpretation we used the preliminary labeling to provide a general understanding of the characteristics of each cluster and to determine which classes have two or more disparate land cover classes represented within their spatial distribution. Table (1) describes the result of labeling. This result gives us a primary idea about the land cover types and their characteristics, this will be taken in consideration in the supervised classification.
Table (1)
| Cluster number |
Land cover class |
| 01 Water |
bodies |
| 01 |
Sea water |
| 03 to 29 |
No specific class |
| 30 |
Wetland, salt soil |
| 31 |
Wetland, sand |
| 32 |
Wetland |
| 33 |
Sand |
| 34 |
Sand, rocks |
| 35 |
Rocks, sand |
| 36 |
Rocks, sand, dry land, range land |
| 37 |
Sand, dry land, semi-dry land |
| 38 |
Sand, dry land, range land, semi-dry land |
| 39 |
Range land, irrigated areas, shrubs |
| 40 |
Shrubs, sub-tropical forests |
| 41 |
Irrigated areas, grass crops, forests |
| 42 |
Gras crops, forests, range land |
| 43 |
Grass crops, forests |
| 44 |
Irrigated areas, forests, grass crop |
| 45 |
Sub-tropical vegetation, forest |
Ground truth data collection and analysis
As the collection of ground truth data is a time consuming as expensive, it is decided to collect the ground truth data from the existing land cover maps described in previous paragraph, especially we consider that land cover categories do not change so much at regional level. From the thematic data and Resurs-1 imagery we collected about57 ground truth region which represent thirteen land cover classes. To keep these data with their geographic location (latitude and longitude), a "ground truth map" was created containing thirteen classes of land cover in form of ground truth. The total ground truth regions were coded by numbers, then "ground truth information sources map" was created according to these assigned codes. A test file containing information about the sources of ground truth regions was attached to this map. It is worth while to be noted that this map will be of great utility as reference information.
The analysis of ground truth data consist of creation of diagrams representing the variations of NDVI values through the year for each ground truth data separately. So thirteen diagrams were created to analyze the data. Table (2) summarize the analysis. From this table we got a lot of information. The maximum NDVI values of vegetation ranges from around of 0.13 to 0.33, the other types of land cover ranges from 0.05 to 0.1, sabkhat land cover has a very small value of NDVI around 0.01. We think that the maximum NDVI value related to sand dune (0.1) is due to reflectance forest have the same range of maximum > 0.30, this is reflect the defect of NDVI to separate the intensive cultivated area from the forest areas, this will produce some concussion in the classification. In general the main period of vegetation activities is April to July, except for irrigation, there are tow periods of vegetation activities, one from early March to end of May, and the other one from early July to end to August. The grass and shrubs land cover class has a low NDVI value comparing with other types of vegetation, we think this is due to the type of shrubs in such dry area, the maximum
activity of this class is during October. The semi-dry land covered by basalt and cherts stones have small peaks especially during April, after a rainfall period, these small peaks are targets of the Bedouin people moving in the desert and looking for the bushes for their animals.
Table (2)
| |
MaxNDVI |
MinNDVI |
Periods of Max NDVI |
Periods of Min NDVI |
| Rang land |
0.18 |
0.07 |
May to July |
December to February |
| Grass crop |
0.09 |
0.09 |
April to June + Oct |
December to February |
| Mixed-forest |
0.23 |
0.1 |
May to July |
December to January |
| Irrigated areas |
0.32 |
0.17 |
Mar to May, Jul to Aug. |
November to January |
| Grass, shrubs |
0.13 |
0.06 |
Feb. to Mar, Sep to Nov. |
June to August |
| Semidry basalt |
0.084 |
0.04 |
April |
May to December |
| Semidry chert |
0.1 |
0.08 |
November to January |
September to October |
| Dry land-desert |
0.07 |
0.04 |
January to June |
July to December |
| Sabhat |
0.01 |
0 |
May |
July to September |
| Sand B |
0.1 |
0.083 |
X |
X |
| Rocks |
0.05 |
0 |
X |
May to September |
| Water |
0.55 |
0.03 |
X |
August |
| Water |
0.0 |
0 |
X |
X |
In general all land cover types are of the same range of minimum NDVI values below 0.1 except for irrigated areas, which is independent of the rainfall and continue through the year. The period of the minimum vegetation activities changes in function of land cover type, but generally from November to February. The diagrams of channel 1 and channel 2 shows a good variation between ground truth data, especially the non vegetation land cover features, so these two channels were selected as additional data to help in discrimination on non vegetation land cover classes.
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