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The world’s cultural legacy consists of rich archaeological ruins (monuments and palaeontological deposits that illuminate past life on earth. Unique historical structures tell us how our ancestors lived and worked. But exceptional population growth, coupled with powerful technologies and industrialisation, caused a serious cultural crisis. With every destroyed site an opportunity is lost forever for future generations to be enriched by their cultural history. The biggest fallacy lies in improper mapping where such monuments are depicted with ordinary symbols of presentation and only the famous and well-known monuments are mapped.

Delhi: The Study Area
In Delhi, history is not confined to dusty tombs and archives, but lives in the architecture, in every stone of its buildings, well-preserved old buildings and others in ruins, scattered all over the city. The Delhi Development Authority (DDA) has listed 1151 monuments worth preserving but there is not a single map showing locations of even 10% of these listed monuments.

Objectives
There are three broad objectives of this study:

1. Development of an “Interpretation Key”
2. Mapping of monuments and sites.
3. Comparison of SPOT images and aerial photographs to bring out their advantages and limitations in such studies.

Data products Used
For this study the panchromatic SPOT (10 metres resolution) of 1:25,000 scale of September 1989- Geo-coded sub-scenes: GRS D207/41 Scene centre:
N28-42/E77-03
N28-41/E77-12
N28-33/E77-12
and April 1987, Row: K207, Path: J293 were used. Panchromatic aerial photographs of 1:10,000 scale of the year 1984 for a small sample area were available and were used for comparison.

Definition of Terms
Historical Sites: A historical site is a place where significant past events have occurred or an area containing property employed in, or monuments commemorating, such events. 


Fig. 1: Interpretation key

Historical Monuments: A monument is often defined by law as any immovable property useful in illuminating or interpreting past events (William, 1978). It can, therefore, encompass monuments in the traditional sense (that is, a large stone statue or other artifact serving no function except to commemorate a person, event or idea), site where significant historical events occurred, buildings, or whole districts’ neighbourhood.

Archaeological Sites: An archaeological site is a place where material remains give evidence of past human life and activities. These remains can be movable and can lie above or below the ground.

Difference between Historical and Archaeological Sites and Monuments
Specialists usually differentiate historical sites from archaeological sites by the advent of writing. If a written history exists of the area or building in question; the site or building is considered a historical site building. If the remains were left at a time when writing did not exist or by a culture that did not possess writing, the site or building is considered an archaeological one.

Non- specialists often differentiate a historical site from an archaeological one by its visibility and state of preservation. Most of the structures and artifacts on archaeological property lie under the ground and must be excavated, whereas most of the structures and artifacts on a historical site lie above the ground. Although not always reliable or accepted, this latter distinction is useful for the purpose of this paper.

Development of Interpretation Key Interpretation Key is a very useful clue and tool for identification and image interpretation. Most questions about classification, interpretation and delineation are answered by the key. Here an attempt has been made to develop a key for interpretation. After the visual image analysis of the panchromatic SPOT image, a key has been prepared which is shown in Fig.1. The explanation of each key is given here: 



Methodology Adopted

• A definite systematic approach of study is required for such investigations and in this study the following methodology has been followed.
• An interpretation key was prepared first for interpretation purpose.
• After preparation of the key, the SPOT image was visually interpreted monoscopically and various monuments and historical sites were delineated on the image using the interpretation elements- shape,size,pattern and tonal variation.
• All these delineated polygons were transferred to a base map (Fig 2).
• Based on the available aerial photographs of a small area, the sample area was marked on the image and the base map.
• Aerial photographs were interpreted stereoscopically for comparison purpose, that is, to find out the potentials and limitations of imagery vis-à-vis aerial photos for such studies. In this process, a few more monuments were interpreted which were not identifiable on the satellite image. The monuments and sites were transferred to the base map and are shown by different cartographic symbols within the photo coverage area marked with broken line (Fig-2).
• A limited field check was carried out to verify the accuracy of photo interpretation, image analysis as well as of key.
• No statistical test was carried out to find the percentage of accuracy because in such studies this has got very limited significance.

Restrictions
At the identification, delineation and fieldwork stages various restrictions were taken into account, like:

• On 1:25,000 scale SPOT image monuments on 75m x 75m size on ground can be identified, i.e., 3mm x 3mm on image. But for delineation purpose (for cartographic ease) 4mm x 4mm was kept. In the case of aerial photographs of 1:10,000 scale 50m x 50m can be identified easily with 3mm x 3mm delineation unit.
• Various small structures within the built-up area were not considered because they might be non-historical religious places and buildings in use.
• At the time of delineation of isolated structures, some excess area around them was delineated for the purpose of cartographic ease.
• Buildings built by Britishers were excluded after field work because they were considered as non-historical for this work and as part of designed and planned cityscape except Coronation Pillar and its adjacent park. But any building built on any historical place was taken as a historic site rather than a building.
• After the final mapping, it was decided not to do the labelling because the names of many lesser known and unknown monuments were not available

Discussions

• Four elements of image interpretation viz. pattern, shape, tone variation, and size played a significant role in identification of monuments and sites.
• Shape of the objects combined with size played a significant role in interpretation. Since people build things as square, rectangular, circular or straight lines, features of this type having considerable size were identified easily.
• Pattern or repetition is a characteristic of many man-made features. Since cultural features consist of straight lines or other configuration, this helped in interpretation and delineation of monuments.
• In the case of panchromatic image, tone variation is a very vital element especially in built-up area. If the tonal variation with surroundings is poor, then many buildings and sites may merge with the surroundings.
• Appropriate scale for identification of buildings and sites for SPOT satellite image should be 1:10,000 or more. But keeping in view the inherent weakness of spatial resolution and pixel size, it is not possible.
• Development of modern buildings and dense residential colonies near historical monuments and sites creates difficulties in identification and idealisation.
• Isolated monuments within built-up area were difficult to identify unless the size of monuments was big enough. But when isolated monuments of smaller size are located in open spaces or green areas properly landscaped, then identification was rather easy.

Conclusion

Two inherent properties of remote sensing image make the data derived from it extremely useful in building a broad induction base:

• The wider environmental perspective afforded by remote sensing techniques over more traditional ones on the ground surveys.
• The permanent nature of such imagery provides a fixed time space perspective of ever- increasing historical value.

The role of large scale vertical aerial photographs is undisputed in such studies because on 1: 10,000 under magnification objects of 0.5 m can be identified, i.e, a spatial resolution of 1m is best suited while resolution and scale of the existing satellite images for this kind of study pose problems. (Excerpts from Photonirvachak, Vol. No. 20, no. 2 & 3, June & Sept. 1992 )