7.1: Topographic Surveys
1. Transit-stadia/theodolite-stadia method 2. Plane table 3. Total stations 4. Photogrammetry 5. Global positioning system.
5 Methods of obtaining topography
Template method
Consists of a series of equally spaced parallel lines that are drawn on transparent film or paper using any specified scale. Series of converging lines may also be graduated and used for interpolating contour lines between plotted elevations.
Approximate contour
Contour accuracy cannot be definitely determined. The area may be inaccesible on the ground or it may be difficult to interpret contours from aerial photographs when heavy cloud or shadow occurs. Map makers has to make an educated guess.
Topography
Defined as the shape, configuration, roughness or three dimensional quality of earth's surface.
Interval
Difference in the values between the lines.
Engineers Planners Foresters Geologist Archeologist Geographers Scientist
Disciplines that applies topography
Depression contour
Drawn to show low spots as excavations around which contour close.
Form line
Drawn when not enough vertical control is available or when the surface is too irregular or intricate contour.
Controlling point method
Elevations are determined for key or controlling points and the contour lines interpolated between them.
Spot heights
Elevations of important features and critical points such as road junctions, peaks, summit sags and highway crossings are indicated in the map
Fifth line
Every _______________ is given a thicker line to make elevation easier to find.
Engineers and planners
Find efficient locations for highways, railroads, canals, pipelines, transmission lines etc.
1. Index contour 2. Intermediate contour 3. Depression contour 4. Supplemental/ Auxiliary contour 5. Approximate contour
Five different types if contours
Purpose of the map
For the design of detailed engineering constructions or for the measurements if earthwork quantities, close contouring will be required.
Variable scales.
Has a graduated springs which when stretched, also allows convenient marks to fall on the known Elevation.
Gentle slope
In hachures, the lines are fine and widely spaced
Steep slope
In hachures, the lines are heavy and closely spaced.
Contour lines for hills
Inner loops are higher in elevations with outer loops.
Contour lines for ditches
Inner loops are lower in elevation with outer loops higher.
Feathering
Intermediate contours are so closely spaced as to nearly unite or merge into a single line, it is standard practice for readability not to portray the lines for short distances.
Geologist
Investigate mineral oil, water and other resources.
Hachures
It gives a general impression of the configuration of the ground, they do not provide numerical values such as elevations of tye ground.
Break lines
It marks the bottom or top a hill or ditch
Light detection and ranging
LiDAR
Scale of the map
The contour interval should be in inverse ratio to the scale of the map. If the map scale is reduced, interval must be increased, otherwise lines are crowded.
Checkboard Method
The desired area is divided into squares or rectangles and the elevation determined in the corners of each of these figures.
Intermediate contour
The four lighter weight contours found between the index contours. They conform to the contour interval specified for the map.
Plane table
The measurements were made in the same way as the transit-stadia method, but the data was plotted in the field on paper that was attached to a drawing board mounted on a tripod.
Relief or terrain models.
The model is a representation of the terrain done in three dimensional aspect to suitable horizontal and vertical scales. (Miniature of the terrain)
Subaqueous Contour
The portrayal of the relief of the ground underwater is done by using depth curves or blue lines.
Form lines
Usually represented as dashed lines and do not portray any actual elevations.
1. Location of property 2. Direction of north meridian 3. Available access to property. 4. Information concerning property corners and monuments. 5. Sufficient elevations to plot contours. 6. Locations, sizes and descriptions of buildings. 7. Locations and sizes if any roads.
7 Checklist of items to be included on a topographic map.
1. Spot heights 2. Hachures 3. Form line 4. Contour lines or Isohypse 5. Layer tinting system 6. Shading 7. Relief or Terrain Models 8. Ridge and Stream Lining 9. Subaqueous contours
9 different methods in representing relief
Nature of terrain
A rugged terrain will require a larger interval than gentle rolling country.
Layering tinting system
A scale of graded color tints or a system of different colors is used to show different zones of elevations.
Total stations 20-40 acres
A very large percentage of topographic surveys today are made with total stations. The instrument which is used for almost all topographic surveys of small areas. Up to ___________________
Index contour
As a convenience to scaling elevations and to provide ease and speed in reading contours, shown by a heavier line at regular intervals on a topographic map.
Global positioning system
Become the common tool of today's surveyor. Combined with a realtime or post-processing error methods, highly accurate surveying and mapping results can be rapidly obtained.
Transit-stadia or theodolite-stadia method
Classical or historical method of gathering topographic data, the necessary measurements were made in the field, recorded in the field and book and then plotted on paper in the office.
Layer tinting system
Color tints when used in conjunction with contour lines give pictorial effect by accentuating the areas of different elevations.
Layering tinting system
Common use for aerial navigation charts and on a small scale maps of the world.
Foresters
Locate access road or haul-roads, fire control routes observation towers.
1. By estimation 2. Rubber band method 3. Analytical method 4. Triangle and Scale Method 5. Template method.
Locating contour lines
Topographic maps
Made to show this information, together with the location of artificial and natural features of the earth, including buildings, highways, stream, lakes, forests and etc.
Ridge and stream lining
Mainly used to emphasize the location of low and high ground. This method is especially used in aerial photographs.
Rubber band method
Method of locating contours with the aid of rubber band, it should be sufficiently wide such that graduations at equal intervals can be marked on it to form some kind of elastic scale.
Contour lines
Most widely used method of quantitatively representing hills, mountains depressions and ground surface undulations on a two dimensional sheet of paper.
Tracing contour method.
Occasionally used when the transit-stadia method was common with these procedure. A number of points were located whise elevations were equal to those of desired contour.
Shading
Old method used to show relative elevations on map.
1. Point density allows an accurate plot of contour lines. 2. Diff. Conditions call diff. Point densities 3. Points should be chosen along hills to make break lines 4. Point can also be uses to locate monuments in the field
Point selection for topographic surveys:
Monuments
Points should also be placed around prominent objects that di not move
Relief or terrain models.
Provide the most striking and realistic expression of topography.
Spot heights
Provides definite and precise information regarding elevation.
1. Relative cost 2. Purpose of the map 3. Nature of terrain 4. Scale of the map.
Selection of contour intervals
1. Checkboard method 2. Controlling point method 3. Tracing contour method
Selection of points for topographic map
Hachures
Series of short lines drawn in the direction of slope.
Contour Lines or Isohypes
Shown on maps as the traces of level surfaces of different elevations. Contains the shape of various topographic features.
Relative cost.
The smaller the interval, the greater is yhe amount of fieldwork. The cost of the map will be higher as the contour interval is reduced.
Form lines
These lines are sketched from visual observation from inadequate or unreliable map sources.
Supplemental/ Auxiliary Contour
They are drawn as dashed lines or lines of dots that begin and end when they approach the areas where the regular contours close on each other.
Relief or terrain models.
They are made from wax, clay, plastic, cardboard and other materials and shaped to agree with actual terrain.
Cartographers
They are the designers of contour map.
Index contour
They are usually drawn every fifth contour and carry the contour number or elevation designation.
Shading
This method is accomplished by correct placement on the map of different shades of gray tints.
Analytical method
This method is suitable for drawing large scale maps also, it is assumed that the slope between two adjacent established points is uniform.
By estimation
This method is suitable for small scale maps where the ground form is not too irregular, wherein contour lines can be located with the given two contour elevations.
Triangle and scale method
This method provides accurate and rapid procedure for interpolating contour lines in which mathematical computations are eliminated.
Computer-aided contouring
Today programs are available to perform the necessary calculations and plot the contour lines and other details quickly and accurately with personal computers.
Photogrammetry
Used for topographic maps of larger areas.