LAB 4: PLANE TABLE MAPPING AND TOPOGRAPHIC PROFILE FROM
TOPOGRAPHIC MAP
OBJECTIVE:
-To relate the 2D representation of topography shown on topographic maps to field features.
-To produce a simple map and introduce the ideas of scale (graphic and representative fraction) and reinforce the use of direction (points of a compass and azimuths).
-To examine topographic changes resulting from volcanic activity.
Commonly when undertaking fieldwork in physical geography, it is necessary to construct a simple map to show the relation of the objects under study. For example, mapping a slope on which you are undertaking a soil or vegetation study to show the location of the soil pits or vegetation quadrats, locating the landforms on a river's floodplain, etc.
Plane table mapping is a method of mapping that can be undertaken with a minimum amount of equipment and walking. If done carefully, it produces a surprisingly accurate map without using sophisticated measuring and surveying tools and can be done by one person, an obvious advantage.
In this exercise you will learn some of the basic principles of mapping by constructing a simple map of Cavanaugh Hall courtyard. You will focus on map scale and directions. In lab 3, we have constructed a contour map using spot elevation. In this lab, you will also learn how to work with maps already produced, topographic maps which use contour lines.
I. DISTANCE AND MAP SCALE
Maps are scale models of the area they represent. The scale on a map provides the map user with a ratio by which all distances on a map must be multiplied in order to determine the actual ground distance. The scale represents the relationship between a distance on a map and an actual distance on the field.
Map scales are commonly expressed in one of two ways:
1. REPRESENTATIVE FRACTION: This is a ratio scale. A representative fraction, or RF, shows the relationship between one of any unit on the map and one of the same unit on the ground. RFs may be shown as an actual fraction (1/24,000) but are usually written with a colon (1:24,000).
EXAMPLE:
1:100
This example can be read as one unit on the map represents 100 of the same units on the ground (i.e., one inch on the map represents 100 inches in the field; 1 cm represents 100 cm, etc.). The RF is versatile because you are not tied to any specific units.
Rules:
i) The number on the left side of the colon is always a 1
ii) Numbers on both sides of the colon are always in the same units (it doesn't matter what the units are: inches, feet, meters, miles, etc.)
2. GRAPHIC SCALE
A graphic scale is always shown as a calibrated bar line that shows directly on the map the corresponding ground distance:
By measuring a distance on the map and then lining your ruler up against the graphic scale, the ground distance can be determined. The advantage of the graphic scale is that if the map is ever Xeroxed and enlarged or reduced, the graphic scale is adjusted at the same time and a new scale does not have to be calculated.
3. LARGE AND SMALL SCALE MAPS
A large scale map is where the RF is relatively large. A 1:1200 map is larger scale than a 1:1,000,000 map. The 1:1,000,000 map would usually be called a small scale map. This is true even though the 1:1,000,000 map would show a much larger area than the 1:1200 map.
Large Scale: 1:25,000 or larger
Medium Scale: 1: 25,000 to 1:1,000,000
Small Scale: 1:1,000,000 or smaller
II. TRIANGULATION
This is the method for locating and mapping locations by shooting azimuths at a landmark from 2 or 3 locations (see figure below). Note that by using this method you do not have to take distance measurements to those landmarks. We will review this principle in class.
