554 
•mmmhmhmbh 
Intervals of 2 in and 4 m, respecitvely. Apart from 
topographic details various cultural and landslide 
details were also mapped. These included old and 
fresh landslide scars, fracture lines, fissures, 
broken ground, houses, vegetation. The stereo-pairs 
were found to be highly useful for a comprehensive 
evaluation of the land-slides and significantly 
helped in a quick assessment of the slide areas. 
4. DIGITAL TERRAIN MODELLING 
The significance of morphomertic and thematic maps 
in the comprehensive evaluation of the project 
areas, particularly those related to geo-environmen- 
tal aspects, hardly need any emphasis. In case 
of Nilgiris landslide investigation the 'Surface 
Area Ratio' maps of the two areas were prepared. 
These established a strong correlation between 
the slope parameters and the incidence of landslides 
and thereby helped in identifying safe and hazardous 
areas. Further, erosion, geomorphological and exist 
ing landuse maps of the areas were also prepared. 
Integration of the morphometric and the thematic 
maps led to a comprehensive evaluation of the areas 
(Agarwal & Sharma 1982). 
Morphometric mapping requires topographic data 
in a digital form at desired intervals, depending 
upon the accuracy requirements and the surface 
irregularities of the terrain. Computer assisted 
analytical stereo-plotters in fact record the topo 
graphic data in a digital form. Also, they hold 
great promise in simultaneously recording the the 
matic data in a coded form, as well. Thus if an 
analytical stereo-plotter is used in place of an 
analogue stereo-plotter then apart from topographic 
base maps, morphometric and thematic maps can also 
be produced simultaneously, either independently 
or in combination. 
DISCUSSIONS 
Though the science of photogrammetry owes its origin 
to the terrestrial photogrammetry, it was soon 
overtaken by aerial photogrammetry due to its obvi 
ous advantages in quickly covering larger areas. 
However, terrestrial photogrammetry maintained 
its relevance and advantages in certain disciplines 
like glacier mapping, and surveys of steep inacce 
ssible or remote areas. In snow mapping the signifi 
cant advantages of the technique has clearly been 
demonstrated by Blyth et.al. (1974). 
Inspite of the increased applications of terres 
trial photogrammetry, particularly in Architectu 
ral engineering and various other fields, its appli 
cation potential in geo-scientific investigations 
has not yet been fully exploited. In this context 
the case histories cited, amply demonstrate the 
potential and advantages of this technique, in 
geo-scientific studies. On account of the numerous 
advantages of this technique over conventional 
survey, the latter appears obsolete in comparison. 
The technique holds great merit in monitoring dyna 
mic features, as repeat photography can easily 
be carried out from fixed camera stations, at desi 
red time invervals. 
While preparing maps by this technique on stereo 
- plotters the ratio between the model scale to 
the map scale is so selected that the maps prepared 
have a plan accuracy of 0.5 mm on the map scale, 
and the height accuracy lies within a quarter of 
the contour interval. This is as per the accuracy 
limits followed by the Survey of India (Agarwal 
1974:78). Thus, if features to be monitored show 
variations larger than 1 mm on plan scale, these 
can be resolved through comnparison of different 
maps. In case monitoring of smaller variations 
is required then either an analytical or semi-analy 
tical approach (Marzan & Karara 1976) is to be 
applied. 
An analytical stereo-plotter, unlike an analogue 
one, has no constraints of photographic parameters, 
and hence it is more versatile, besides being more 
accurate too. Another aspect where it has signifi 
cant advantage over analogue stereo-plotter is 
in digital terrain modelling which holds great 
promise for simultaneous topographical, morphometric 
and thematic mapping. However, it is much more 
sophisticated, costly and would involve the inter 
action with experts for its operation. Accounting 
various aspects, Marzan & Karara (1976) have opined 
that if the maps produced by analogue stereo-plotter 
meet the accuracy requirements for a particular 
work, then the anologue method is the simplest, 
most straightforward and perhaps the cheapest method 
to employ. 
It may be summarised that the technique of terres 
trial photogrammetry is unique for preparation 
of large scale topographic and thematic maps, for 
a comprehensive evaluation of the project areas 
in the shortest possible time. Simultaneous photo 
interpretation, field checks and ground truth 
data collection considerably expedites the work. 
However, it would be relevant to mention that 
the suitability of the technique for a given area 
largely depends upon the availability of suitable 
locations from where stereo-photographic coverage 
of the area could be obtained. 
REFERENCES 
Agarwal, G.C. 1974. Photogrammetric surveys, their 
planning and costing, Tech. Public No.7401, 
Survey of India, Hyderabad. 
Agarwal N.K. 1985. Mapping of inaccessible escarp 
ment slope of left abutment of Supa Dam, Karna 
taka, utilizing stereo-terrestrial photogramme 
try, Jr. of Engineering Geology Vol.XIV, Nos. 
1 & 2, ISEG. 
Agarwal, N.K. & Srivastava, G.S. 1982. Geomorpho 
logy and Environment of Coconut Island, South 
Kanara distt., Karnataka, Proc. Symposium on 
Resources surveys for Landuse planning and Envi 
ronmental Conservation, ISPI & RS, Dehradun, 
India. 
Agarwal, N.K & Sharma. R.P. 1982. Terrestrial 
photogrammetric evaluation of slide areas, Nil 
giris dist., Tamil Nadu, Proc. 4th International 
Congress, IAEG, India, Vol. III. 
Blyth, K. Cooper, M.A.R., Lindsey, N.E. Painter, 
R.B. 1974. Snow depth measurements with terres 
trial photos. Photogrammetric Engineering, Vol. 
XL, No.8. 
Marzan, G.T. & Karara, M.M. 1976. Rational design 
for close range photogrammetry, photogrammetry 
series No. 43, Deptt. of Civil Engineering, 
Univ. of Illinois, Urbana, Illinois, 61891. 
Manual of Photogrammetry 1980, Fourth Ed. Am. 
Society of Photogrammetry, Falls Church, U.S.A. 
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