Forest Mapping

We have already seen about the area under forest in India which is abysmally low and better than the not so naturally well endowed neighbors. Here we will be see how science and satellite imagery is employed to map a forest.

Forest Survey of India (FSI) started using satellite date for assessment of forest cover since 1986 with a resolution of satellite data being(pixel size 23.5m X 23.5 m), scale of interpretation (1:50, 000) and the technique employed for image processing. Satellite data is procured from the National Remote Sensing Agency (NSRA), Hyderabad in digital form. Data is normally collected to the period from October to December when the cloud cover is low and the deciduous trees still have leaves to provide satisfactory reflectance for the satellite sensors.

Using Digital Image Processing (DIP) software, digital data from satellite is downloaded to the workstation. Radiometric and contrast corrections are applied to remove radiometric defects and for improving visual impact of the False Color Composites (FCC). Geometrical rectification of the data is carried out with the help of scanned SOI toposheets. Based on tone and texture of the forest cover areas are delineated. Interpretation of forest cover for the whole country is done at 1:50,000 scale using polyconic projection. Normalized Difference Vegetation Index (NDVI) transformation is also used for density classification of forest cover. Areas of less than 1 Ha, whether classified as forest within non-forest areas or blanks within forested areas, are excluded by clustering pixels and merged with the surrounding class.

Highly degraded forest or wastelands with stumped trees having canopy density less than 10 percent are classified as scrubs, a category of non-forest cover. Shadow areas in the scenes are treated separately. Density in shadow area is either based on ground information or is assigned due to their unique signature along the coastal areas. This is then followed by extensive ground verification and all the necessary corrections are subsequently incorporated.

But even with remote sensing technology there are some limitations in assessing the forest cover. Young plantations and species having less chlorophyll contents in their crown do not give proper reflectance and as a result are difficult to be interpreted correctly. So will the cloud cover making considerable details becoming obscure. Gregarious occurrence of bushy vegetation and certain agricultural crops, such as sugarcane, cotton, often pose problems in delineation of forest cover, as their reflectance is similar to that of tree canopy.

The post concludes by bringing out the difference between forest cover and forest area

A land may be recorded as forest cover and under management of forest department but may not have any discernible forest cover. On the other hand, all wooded lands or plantations, delineated as forest cover from satellite data may not be legally recorded as forest area as these could be private plantations or institutional wood lots. Although, majority of forested lands happen to be within legally recorded forest areas, all the changes taking place in the forest cover is not necessarily due to changes in the forests managed by the forest departments.

From policy and planning point of view to know the extent and quality of forest cover within recorded forest areas and outside it, the latest geo-referenced forest maps for the whole country showing the latest boundaries of recorded forest areas of 1:250,000 scale should be available using GIS tools for the purpose. On an average, at least 20 percent area within the reserved forests is without forest cover.

*The reference for this post is currently unavailable as I had come across it during one of my reading sprees and didn’t have the foresight to note down the source.

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