DIFFERENCES BETWEEN A PLAN AND A MAP

SUBJECT : SURVEYING

Plan

A plan graphically represents the features on or near the earth’s surface as projected on a horizontal plane. As a plan represents the area on a horizontal plane, the horizontal distances are measured between various points on the earth’s surface. If the distances are measured on slopes, these are always reduced to equivalent horizontal distances. The horizontal plane is represented by the plane of the drawing sheet on which the plan is drawn to some scale. However, as the earth’s surface is curved, it cannot be truly represented on a plan without distortion. In plane surveying, as the areas involved are relatively small, the earth’s surface may be regarded as a flat plane without much error. The plan is, therefore, constructed by usual orthographic projections. A plan is drawn on a relatively large scale.

Map

If the scale of the graphical projection on a horizontal plane is small, the plan is termed map, In addition, a map generally shows some additional features, such as reliefs, hachures and contour lines to indicate undulations on the ground.

As the terms ‘large scale’ and ‘small scale’ are not well-defined, the distinction between a plan and a map is rather arbitrary. One cannot definitely say when a plan becomes a map. Generally, large scale representations of small areas in engineering surveys are called plans, whereas small scale representations of large areas are called maps. The two terms are used synonymously in this text.

SCALE OF A MAP

Scale of a map is the ratio of the distance marked on the map to the corresponding distance on the ground. As the areas involved are rather large, it is essential to select a suitable scale for representing the area on a map. Selection of the scale depends upon the purpose, size and the required precision of plotting. In general, the scale should be large enough so that in plotting or in scaling distances from the map, it should not be necessary to read the scale closer than 0.25 mm or so. A good draughtsman can plot a length to accuracy within 0.25 mm.

On the other hand, the scale of a map should not be larger than what is required to represent the features with sufficient precision, For example, if a survey is conducted to an accuracy of 0.25 m, a distance of 0.25 m on the ground can be taken as 0.25 mm on the map. This corresponds to a scale of 1 mm equal to 1 m or 1 in 1000. It would be a wastage of time and effort if the scale is chosen much larger than this scale.

Scales are generally classified as large, medium and small as under

Large scale: 1 cm = 10 m or less than 10 m

Medium scale: 1 cm = 10 m to 100 m

Small scale: 1 cm = 100 m or more than 100 m.

For most of engineering projects, the scale varies from 1 cm = 2.5 m to 100 m. Small scale topographical maps are usually drawn to a scale of 1 cm = 1 km. A scale of 1 cm = 5 m to 50 m is generally used for plans prepared for subdivision of land. Table 1.1 gives suggested scales for different types of surveys, depending upon their purpose. It may be noted that the scale is said to be large when the size of the figure is large.

TYPES OF MEASURING SCALES

ERRORS IN SCALE (SURVEYING)

PRIMARY DIVISIONS OF SURVEYING

FUNCTIONAL AND INSTRUMENT BASED CLASSIFICATION OF SURVEYING

PRINCIPLES OF SURVEYING

BASIC METHODS, INSTRUMENTS & MEASUREMENTS IN SURVEYING

REPRESENTATION OF SCALE IN SURVEYING

INTRODUCTION & HISTORY OF SURVEYING