E. V. Anoop
College of Forestry, Kerala Agricultural University
Vellanikara, Thrissur 680656
GIS allows us to study and understand the real world process by developing and applying manipulation analysis criteria illuminate under lying trends in geographic data, making new information available. A GIS enhances this process by providing tools which can be combined in meaningful sequence to reveal new or previously hidden relationships within or between the data sets, thus increasing better understanding of the real world.
A continually varying surface can be represented by isolines (contours), and these contours can be effectively regarded as sets of closed nested polygons. While, sets of isolines are very suitable for the display of continually varying surface they are not particularly suitable for numerical analysis or modeling. So other methods have been developed in order to be able to represent and to use effectively information about the continuous variation of the attributes (usually altitude over space).
Any digital representation of the continuous variation of relief over space is know as digital elevation model (DEM). A digital elevation model is an ordered array of numbers that represent spatial distribution over elevations above some arbitrary datum in the landscape.
There are various structures for DEM in use. As such none of the data structures satisfies all requirements. One has to look for the suitable type depending upon the purpose. Commonly used structures for DEM are: (a) Line Model (b) Triangulated Irregular Network (TIN) (C) Grid Network.
The Triangulated Irregular Network (TIN) is a system designed by Peuker et. al. (1986) for digital elevation modeling. The TIN model is a vector topological structure similar in concept to the fully topologically defined structures for representing polygon networks with the exception that the TIN does not have to make provisions for islands or holes. The TIN model regards the nodes of the network as primary entities in the database. The topological relations are built into the database by constructing pointers from each node to each of its neighboring nodes.
By analysing a digital elevation model, highly useful products can be derived, which are application in various fields. Few of such products are contour maps, line of sight maps, line of sight maps volume estimation by numerical integration, maps of slope, convexity, concavity and aspect, shaded relief maps, drainage networks and drainage basin delineation etc.
The objective of the present study was to prepare a 3-dimensional Geographic Information System (3-D GIS) of the Indian Institute of Remote Sensing (IIRS) campus located at Dehradun. It was also aimed at producing a DEM of the campus and conduct various surface analysis and produce products such as slope map, contour (interpolated) map, aspect map, hill shade map etc. A profile analysis for a proposed road in the campus was also envisaged.
The study area, IIRS campus is located at 4 Kalidas road Dehradun, in Uttaranchal State. The total area of the campus is 10Ha. The topography of the area is plain with few gully structures. The vegetation is of the scrub type with few planted avenue trees and scattered forest tree species and bamboos.
Materials and Methods
Procurement of data
The 3D model of the campus was prepared, based on a survey map of the campus. Various features such as contour lines, buildings, roads, walls, fence etc. were available from the map. Other details such as distribution, height and species information of trees height information of various structures were collected directly through the field study. Digital photography of buildings and procurement of video files for the purpose of hot linking were also done.
Interpretation of data
For the interpretation of the data, a preliminary investigation was conducted in the field. Accuracy of various themes were checked and necessary corrections made. As the survey map was prepared during 1992. Many new additional structures were added to existing map. As the survey map was of multi-featured nature, it was necessary to isolate various themes through tracing, for carrying out proper digitization.
Data entry and digitization
In order to obtain accurate results, tablet digitization by using PC/ARC INFO version 3.5.1 was followed. Various layers that were digitized are as follows: 1. Contours 2. Buildings 3. Roads 4. Walls 5. Fence 6. Lawn 7. Trees 8. Drains 9. Power Line 10. Telephone Line and 11. Internet Line.
After the digitization of various layers, the planar map was transformed from screen coordinates to UTM, and was exported to Arc View 3.1 as arcshape files. Further processing was done in Arc View. Attribute data such as contour heights, building heights and names, wall heights, tree species and heights were added to the data files of the respective themes. In addition, new fields for images and videos were added to the data file of the buildings theme for the purpose of hotlinking. For hotlinking video files new scripts were written using avenue. The Triangulated Irregular Network (TIN) was created from the contour map. Various thematic layers such as roads, walls, fence, lawn, trees, drains, power line, telephone line, internet line etc. were then overlaid with the TIN.
Final results were obtained by the thematic overlays on TIN features, as 2D themes. Digital Elevation Models were obtained as 3D themes by the overlay of extruded elevated features such as buildings, trees etc. over the TIN. Various GIS surface analysis such as calculation of area and volume of DEM, line of sight analysis, measuring and profiling height along a line were carried out and contour maps, aspect maps etc. were constructed.
Figure 1: 3-D view of IIRS campus
Results and Discussion
Surface area was measured along the slope of a surface (TIN of the campus contour map), taking height into consideration. The base height above which the surface area was calculated was taken as 677 m, the lowest contour for the entire study area.
Volume reported represents the space that’s above the plane and under the surface. The base height above which the volume was calculated was taken as 677m, the lowest contour for the entire study area.
The surface area and volume calculated for the TIN is as follows :
Planimetric area Surface area Volume 94437.95m2 99009.57 1402126.28 *Calculated above base height of 677m
Volume of water body
In the present study, a water conservation structure (check dam) was digitised at a suitable location on the down stream end of a gully and then added as a 3-D theme over a selected subset of the terrain TIN surface. The volume of the space between the TIN surface and the horizontal plane passing through the top of the check dam was calculated to measure the total volume of water that can be retained inside the structure. The planimetric area, surface area and volume is given below.
Planimetric area Surface area Volume 2139.40m2 3184.58 m2 12011.02 m2 *Calculated above base height of 688m
Figure 2: View of the proposed water conservation structure
Line of sight from a point near the GID building to a point near the IIRS gate was determined. The visible portions of the line to the target was added in green; and the non visible portion is in red. The Arc View status bar at the bottom of the application window indicated that the target location was invisible. The first obstruction point along the line of sight was shown as a blue point graphic and the xy coordinates of the obstruction was reported in the status bar.
Measuring and profiling height along a line
In the present study, the proposed straight road from the GID building to type E quarters over the gully gave the variation in height and terrain along the straight road.
Measuring the steepest path downward from a point
The steepest path downward from a point helps to find out the water flow characteristics over a terrain as water generally follows the drain along the slope. Several points located at higher elevations within the campus were selected and the steepest path downward from the points were found out using the steepest path tool in surface analysis. The following figure shows steepest paths downward from different points selected within the campus.
The author acknowledges the Indian Institute of Remote Sensing, Dept. of Space, Dehradun for rendering necessary assistance and guidance in carrying out the above work using the facilities of Institute.