Introduction to CartographyGEOG 2016 E

Lecture-4Sources of Data

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.

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

Survey Types• Photogrammetry

– 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

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

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

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

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

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

Horizontal Distance

ZenithAngle

VerticalAngle

Taping Error

• Instrumentation 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

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

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

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

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

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

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

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

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

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

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

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

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

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

GIS Data Structures• A GIS has two main data structures: vector

and raster• Vector is made up of points, lines and

polygons

GIS Data Structures

• Raster or grid is made up of pixels on computer screen