GIS in forestry: A bottom to top approach for high accuracy and large-scale integration of geo-spatial data
Use of GPS in Forest Traverse
In forest areas, in almost all the states in the country, survey is still carried out in the traditional manner with chain and compass. As already stated, the process is not only man-power intensive and time consuming, but also prone to large subjective and objective error, especially in hilly terrain. Use of sophisticated survey equipment such as EDM is yet to dawn in this sector. In the present scenario, GPS appears to be a viable alternative for accurate forest traverses.
The use of GPS in forested terrain has its own sources of error. In closed canopy conditions, either the satellite radio signal may not reach the GPS antenna, or the signal may get attenuated. The canopy effect may lead to loss of lock. In such circumstances, the idle time may increase considerably. However, as per available literature, performance of DGPS in static mode is very satisfactory. While using DGPS in static mode for accurate boundary description, canopy lifting/opening may be resorted to so as to enable the GPS antenna to lock to minimum four satellites.
The advantages of GPS traverse over conventional survey are many. Operational time, manpower and cost are reduced considerably. Closing error which is so common in, compass and chain survey, does not exist at all in static GPS traverse, as because one directly gets the nodes of the polygon. In case of a large traverse, time is further saved by downloading the data into computer.
Researchers at Ohio State University have developed the concept of GPSVan, a mobile mapping system. Such concepts can be easily put to utility in forest areas without involving any overhead costs. GPS can be mounted on a vehicle and forest areas such as roads, fire lines, compartment and reserve boundaries can be surveyed and mapped. Mapping of streams and channels can be made possible by mounting the instrument on a motor/country boat, Thus, with the help of GPS, locations of watch towers, camp sites, depots, administrative units, plantation sites, bio-diversity hot-spots, migration route of animals also can be mapped with great accuracy and little operational cost.
By using DGPS in static mode, it is also possible to reconstruct forest boundaries in terms of absolute co-ordinates (say in WGS 84). These co-ordinate pairs, then, can be directly used to position the reserves accurately on the map. Thus, ‘With the help of GPS one to one mapping is established from map to ground and vice-versa. Accuracy of the digital maps will increase further.
In order to use GPS more effectively, it is necessary to have Ground Control Points (GCPs) whose co-ordinates in WGS 84 datum (or in any other accepted system) are known with a high degree of accuracy. In order to facilitate quick mapping, there should be established at least one such GCP in every district. The onus of maintaining such GCPs may be vested in the Survey of India.
For such detailed information, the process of digital mapping must be that of bottom to top approach. Mapping at the ground end would generate large volumes of geo-spatial data, which necessitates the concept of seamless integration of digital maps.
<>bCan Maps be Viewed in Isolation?
The earth’s surface is a vast continuum of various geographic features. Maps represent projections of these features for a particular area on a particular scale. A series of maps may be produced to map the earth’s entire surface. But, paper maps have inherent limitations. To illustrate the case, let us take the example of Majuh, the biggest river island of the world, situated in Jorhat district of Assam. Approximately, it has an area of 1,000 sq. krn. The Survey of India mapped the island on 1:15,840 scale in 1968. For grid referencing, the whole area was divided into grids referenced horizontally as BO to BZ, and vertically I I to 22. Thus, a total of 144 topographic sheets were produced for the entire island, which is a large number.
Maintenance, storage and indexing of such large number of paper maps is not possible in normal office environments. Shelf life of these maps is also very short as they are subject to mutilation and soiling. Therefore, there is no doubt that switching over to digital maps is the only solution. However, digital maps also suffer from certain limitations, Size of these maps is directly related to computer memory (RAM), storage space (disk drive and back up capacity) and processor speed (chip type and its clock speed). A digital map file of size 100MB may take from several minutes to more than a hour just only to load, depending upon the computer configuration. Larger file sizes increase idle time considerably. Thus, digital maps too have limitation on size just as paper maps. A large number of map files of smaller sizes, say I to 30 MB need to be generated. What if these maps cannot be tiled or mosaiced with one another for want of memory and processor speed? The very purpose of digital mapping would be defeated.