Introduction to Cartography GEOG 2016 E

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Presentation transcript:

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 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

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 Zenith Angle Vertical

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

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