1. Introduction to Cartography GEOG 2016 E
Lecture-4
Sources of Data

2. Cartographic Data and Surveying
To make a map we need data.
The cartographic data is generally obtained by surveying.
There are other methods too, such as
Remote Sensing, Census and Sampling
However, surveying is the most commonly used method of gathering data for generating maps.
Surveying is the method of determining the relative positions of points on, above or beneath the earth’s surface.

3. Categories of Surveying
Plane Surveying
Surveying with the reference base for fieldwork
Surface is assumed to be flat horizontal
Generally within a radius of about 20 km, the pull of gravity is nearly parallel – hence horizontal lines can be considered straight
Geodetic Surveying
To determine relative positions of widely spaced points, lengths and directions
Takes earth’s curvature into account

4. Survey Types Photogrammetry Topographic Surveying Route Surveying
By camera or other sensors in airplanes or satellites
Topographic Surveying
Collecting data specifically for making maps
Route Surveying
For civil engineering projects, such as highways, railroads, pipelines etc.
Hydrographic Surveying
To map shorelines and the bottom of water bodies

5. Measuring Distance Linear measurement is the basis of all surveying
Methods of measuring horizontal distance
Rough Measurements: Pacing, Odometer, Tacheometer
Accurate Measurements: Taping, EDM, GPS
EDM and GPS are the most commonly used methods for surveying

6. Taping
Applying the known length of a graduated tape directly to a line a number of times
Steps
Lining in: shortest distance between two points is a straight line
Applying tension: rear point of tape is anchored and tension is applied to the head
Marking tape lengths: each application of the tape requires marking using chaining pins
Reading the tape: the graduated tape must be read correctly
Recording distance: total length must be recorded accurately

7. Types of Tapes and Chains
Gunter’s Chain
66’ long with 100 links with each link being 7.92 inches long
Developed by Edmund Gunter in 1600’s in England
Engineer’s Chain
Same construction as Gunter’s chain but each link is 1.0’ long

8. Types of Tapes and Chains
Surveyor’s and Engineer’s Tapes
Made of ¼” to 3/8” wide stell tapes in 100’, 200’, 300’ lengths
Multiple types of marking and graduation
Available in feet and metric
Invar Tapes
Made of special nickel steel to reduce length variations due to temperature changes
Extremely brittle and expensive
Used mainly for standard comparison of tapes
Cloth, Fiberglass, PVC Tapes
Lower accuracy
Used for measurements of 0.1’ accuracy

9. Slope Measurements Generally, measurements are made horizontally
Often the distance can be measured directly on the slope
However vertical or zenith angle must be obtained
Horizontal Dist. = sin(zenith angle) x slope distance
Horizontal Dist. = cos(vertical angle) x slope distance
Slope Distance
Horizontal Distance
Zenith
Angle
Vertical

10. Taping Error Instrumentation Error Natural Error Personal Error
Tape may be defective
Natural Error
Tape length changes due to temperature, wind or weight of the tape
Personal Error
Carelessness in setting pins or reading tape

11. Transit Transit is the most widely used surveying instrument
Used for measuring horizontal and vertical angles
Can also measure vertical and horizontal distances
Components
Alidade: upper part
Horizontal limb: middle part
Levelling-head: Lower part

12. Total Station
Modern version of transit with an electronic distance meter (EDM)
Electronic/optical instrument
Can measure slope distances from the instrument to a particular point
Extensively used in modern surveying

13. Levelling
Levelling is the determination of the elevation of a point or difference between points referenced to some datum
Terminologies
Datum: Any level surface to which elevations are referenced
Mean Seal Level (MSL): The average height of the surface of the sea for all stages of the tide over a 19 year period at 26 tide stations along Pacific, Atlantic and Gulf
National Geodetic Vertical Datum: Nationwide reference surface for elevations throughout the US

14. Levelling Mostly mean sea level is used
MSL varies along the coast
Pacific is almost 2’ higher than Atlantic and Gulf
The level surface parallels the curvature of the earth
Hence a level line is a curved line
Generally measured with the Automatic or Self-levelling level

15. Differential Levelling
Also called Spirit Levelling
Most common type of levelling
Determines the difference in elevation using a horizontal line of sight and readings on a graduated rod

16. Topographic Surveying
Process of:
determining the positions of the natural and artificial features on the earth’s surface
Determining the configuration of the terrain
Planimetry
Location of features
Topography
Configuration of the ground

17. Topographic Surveying
Scale and accuracy
Depend on the instrument used and method
Presentation method
Contour Lines: Imaginary line on surface of the earth passing through points that have equal elevation
Contour Intervals: Vertical distances between lines
Index Contour: Every 5th contour drawn heavier on maps
Slopes can be obtained from contours

18. Topographic Surveying
Interpolation
Can find elevation of any point
Or find a contour line with known elevation of a point
Contour Characteristics
Each contour must close within a map or outside its borders
Contours do not cross or meet except in caves, cliffs and vertical walls
Contour lines crossing streams form V’s pointing upstream
Contour lines crossing a ridge form U’s pointing down the ridge

19. Methods of Topographic Surveying
Factors that influence the choice of method
Scale of the map
Contour interval
Type of terrain
Available equipment
Accuracy required
Extent of area to be mapped

20. Methods of Topographic Surveying
Cross section
Railroad or highway
Equipment used: transit, tape and level
Trace contour
Drainage or impoundments
Involves finding elevations at different points using transit
Grid
Small areas
The smaller the grid, the better the accuracy
Controlling point
Large area, plane table
Positions and elevations of pre-selected control points
EDM
Radial
Equipment: total station

21. Errors in Topographic Surveys
Improper selection of contour interval
Improper equipment or field method for the particular survey and terrain conditions
Insufficient horizontal and vertical control of suitable precision
Omission of some important topographic details

22. Global Positioning System (GPS)
Developed in early 1980’s by US Department of Defence
Made up of 26 satellites orbiting earth
24 are functional and 2 spare
Each satellite is in a fixed position
Signals from at least 3 satellites are needed for accurate positioning
Receivers with high position accuracy are fairly expensive

23. Geographical Information System (GIS)
GIS are computer programs that allow users to store, retrieve, manipulate, analyze and display spatial data
GIS techniques are now being used in many fields, such as physics and medicine
GIS components
Hardware (computer)
Software
Data
Operator

24. GIS Data Structures
A GIS has two main data structures: vector and raster
Vector is made up of points, lines and polygons

25. GIS Data Structures
Raster or grid is made up of pixels on computer screen