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A Use Satellite Images for Preparation of Base Map

A1 Need for Accurate Base Map for a Development Plan
A Development Plan lays down the framework for future growth, determines the infrastructure development proposals & investments and prescribes the use and development rights for each parcel of land. It is therefore absolutely essential to have a reasonably accurate and updated map to lay the road map for future growth. City Maps are prepared on the basis of maps showing plot details that are obtained from the cadastral maps /Land records available with the District Inspector of Land Records and Revenue Department. These were made during the British times and haven't been systematically updated since then. There are piecemeal changes made by individual plot owners owing to sub division of land or when land is acquired for roads/public projects etc. The information regarding these changes vests with different departments and has not been systematically collated to prepare updated maps. Further new growth and development are not reflected on the maps. Also as these maps are rarely computerized and have been manually traced/replicated over the years, they are even inaccurate geographically. Consequently, they don’t serve contemporary requirements as they are mostly outdated, their generation and maintenance is cumbersome, and they are incompatible with the planning and development requirements of the present times.

However it is not only important to have accurate and up to date Base Maps, but also city maps must be computerized so that layers of information can be attached and spatially represented to support and aid planning decisions. Further, computerization ensures accuracy, makes constant updation easier and makes it possible to view and analyze data at various scales. The first major task in any planning exercise is to prepare an accurate, up to date and computerized Base Map.

A2 Available Methods to Prepare a Base Map for an Urban Area
A geographically accurate Base Map can be generated either by undertaking a detailed topographical survey or by using satellite images along with other sources of information. The salient features and pros and cons of both the methods are briefly described.

A2.1 Topographical Survey
A comprehensive physical survey of the planning area can be carried out using total station survey equipment. Several details can be surveyed ranging from all built features, roads, natural elements, levels etc. This process ensures considerable accuracy however stringent controls have to be ensured while setting the traverse and taking measurements. Such a survey can not only serve the planning objective adequately but also can be later used for detailed planing of infrastructure. Detailed surveys are required while planning and implementing infrastructure works such as roads, drainage, water supply works etc., which need not be repeated. Subsequently the survey information can be used to generate other applications such as assessment of property taxes, mapping of other infrastructure networks for the purpose of maintenance, management, planning etc.

However this requires considerable amount of resources/investments upfront and in most cases wherein the planning agencies don’t have adequate resources a topographical survey is rarely undertaken. Also considerable amount of time is required to carry out a detailed topographical survey especially when there are densely built up areas which is the case with most Indian cities. But this is highly recommended as the investment is justified given the usefulness of such a survey by building centralized databases for all planning agencies, utility agencies etc.

A2.2 Use of Satellite Images
Given the resource constraints, availability of satellite images has made the task of correcting and updating the existing city maps much easier and faster. A reasonably accurate Base Map can be prepared using the satellite images (PAN and IKONOS) as a base and integrating information from various sources such as aerial photographs, revenue maps, SOI sheets, maps from various departments etc. Appropriate corrections are required to ensure geographical accuracy such as geo-referencing and registration of satellite images with topographical sheets. Accuracy can be ensured depending on the resolution of the satellite images used. Certainly the use of such maps is limited as compared to the ones based on topographical surveys.

This methodology has been used for correction and updation of the region and town map of Tirupati Urban Development Authority. The next section describes the process in detail.