Applications
GIS in Forestry - A Bottom to Top Approach
Moreover, an error may not appear at all on 1:50,000 scale, but may become very prominent in a digital map. Such maps containing error may only serve the purpose of researchers and scientists. However, they are not fit for use in the field situation. An error of 10m in on the ground would be represented by 0.2 mm on a 1:50,000 scale map. The drafting pens normally employed for paper maps are of dimension 0.1 to 0.2 mm. Therefore, such an error may escape un-noticed on a paper map. If that error were to run for 40 cm on the map along a line, it would represent 20 ha of actual error on ground. Estimated loss that may occur to the exchequer for such an error could be well above Rs. 10 crores assuming 60% stocking of the area.
The need for such a high level of accuracy did not arise earlier as because administrative decisions were not based on high end technologies. Error in mapping is largely technology related. Error in paper maps may be termed tolerable, but not so at all in digital maps, An inference drawn from a computer based GIS which is not free from error, may trigger a series of undesirable actions in the executive hierarchy of governance.
An Iterative Algorithm for Error Free Digital Mapping The advantage of working with digital maps is that there is always a room for improvement and correction, involving very little cost and time. Such flexibility is not available with paper maps. Data from various sources may be used to correct digital maps.
Modern cartography largely depends upon aerial photography and satellite imageries. It is, therefore, possible to develop in-house cartographic skills to meet one’s mapping and accuracy requirements. In the paragraphs below is briefly given a sketch of mapping requirements from forestry point of view and also how digital maps are being prepared in Assam for various Forest Divisions.
The management of Reserved Forests and other forest areas is based on compartments which are nothing but small sub-divisions of the forest, the boundaries of which are actually laid down on the ground, Each compartment has a unique number. The complete data base of a compartment is maintained in Compartment History. History of a particular compartment would show not only its geology, rock, soil and forest type but also management principles applied to it from time to time. It also contains records of plantation, incidents of fire, any dereservation/disforestation, alteration in boundaries and any other significant incident pertaining to that compartment. These histories are updated during compilation of Working Plans at fixed interval of time.
To build forestry GIS for field application, compartment is the right mapping unit to start with. During the early British period of conservancy, the compartments were duly mapped by Survey of India on 4" = I Mile scale. Now original prints of such maps are hard To come by. Repeated tracing over the past several years have rendered the copies of the maps available today unusable, as they have come to contain many errors.
While preparing digital maps for the Divisions, care was taken to verify the map data from various sources, including field visits. As 4" = I Mile maps were not available or were unusable, there was no option but to use topographic sheets on 1:50,000 and 1:250,000 scale. It is noteworthy to mention here that these scales, in general are considered unsuitable for forest mapping. Reliable old stock maps were used wherever possible.
As Assam has a long international border with Bangladesh, many of the topographic sheets have been frozen by Survey of India. Wherever old sheets were available on 1" = I Mile and I"= 4 Mile, they were also utilized. To put together maps on various scales in order to produce a single map, digital mapping was the only way out.
Digital maps so prepared were subjected to refinement by using IRS IB/IC False Colour Composites. Details such as changed river courses, channels and streams, water bodies, highways and railway lines were lifted from the imageries and incorporated in the digital maps. In case of ambiguity, even IRS IC PAN images were also used to modify the maps further. In cases where ground survey data was available for the Reserved Forest areas, the bearings and distances were plotted in a separate file and subsequently inserted in the main maps. In certain cases where boundaries were not natural, ground survey was resorted to.