Survey
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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 Zenith Angle Vertical Angle Horizontal Distance 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