Geog 458: Map Sources and Errors January 9 2006 Representing Geography.
Introduction to Cartography GEOG 2016 E Lecture-4 Sources of Data.
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Transcript of Introduction to Cartography GEOG 2016 E Lecture-4 Sources 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
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