Contents preface (VII) introduction 1—37

5.3.2. Detailed Survey
The preparation of plans for a large canal project is simplified in a developed area because of the availability of settlement maps (also called shajra maps having scale of 16 inches to a mile i.e., 1/3960 ≅ 1/4000) and revenue records in respect of each of the villages of the area. The settlement maps show the boundaries and assigned numbers of all the fields of the area, location of residential areas, culturable and barren land, wells, ponds, and other features of the area. Usually for every village there is one settlement (or shajra) map which is prepared on a piece of cloth. These maps and the revenue records together give information on total land area, cultivated area, crop-wise cultivated area and the area irrigated by the existing ponds and wells.
With the help of settlement maps of all the villages in a doab, a drawing indicating distinguishing features, such as courses of well-defined drainages of the area, is prepared. On this drawing are then marked the contours and other topographical details not available on the settlement maps but required for the planning of a canal irrigation project. Contours are marked after carrying out ‘levelling’ survey of the area.
The details obtained from the settlement maps should also be updated in respect of developments such as new roads, additional cultivated area due to dried-up ponds, and so on. In an undeveloped (or unsettled) area, however, the settlement maps may not be available and the plans for the canal irrigation project will be prepared by carrying out engineering survey of the area.
One of the most important details from the point of view of canal irrigation is the watershed which must be marked on the above drawing. Watershed is the dividing line between the catchment areas of two drains and is obtained by joining the points of highest elevation on successive cross-sections taken between any two streams or drains. Just as there would be the main watershed between two major streams of an area, there would be subsidiary watersheds between any tributary and the main stream or between any two adjacent tributaries.