Surveying and Digitizing Primary Data Sources n

Surveying and Digitizing

Primary Data Sources n Measurements n n n Remotely sensed data n n Field → surveying Lab (not covered here) already secondary? Creating geometries n n n Definitely in the realm of secondary data Digitizing Scanning

Surveying n Measurements and measurement techniques n n Distances Angles Position determination Applications n n n Traversing and mapping Construction and earthwork Boundary surveys

Definition of Surveying n General n n n To inspect, view, scrutinize, or examine To determine condition, situation, or value Specifically n Science and art of determining relative positions of points above, on, or beneath earth surface

Uses of Surveying n Locate/map resources n Engineering design n n Layout construction or engineering projects Verify performance Acquire reliable data Provide control n Usually for location

History of Surveying n Early applications n n Boundary location Construction Mapping Early surveys limited by technology n n Crude and inconsistent methods Development of sighting devices, standards, …

History of Surveying (2) n Industrial revolution improved surveying n n n Advances in available materials Improvement in tools Electronics revolution fundamental advances n n n Electronic distance and angle measurement Satellite surveying Enhanced processing

Specific Types of Surveying n n n n Property (cadastral) surveying Control surveying Mapping surveying (planimetric or topographic) Photogrammetric surveying Construction (engineering) surveying Route surveying Hydrographic surveying

Surveying Measurements n Two quantities measured in surveying Lengths n Angles n n All measurements are imperfect Errors n Mistakes n

Measurement Errors n Sources of errors n n Personal Types of errors n n n Natural Instrumental Systematic Random Terms used in describing errors n n Precision Accuracy

Idea of Relative Position n Question: Have the points moved? Answer: Relative to what? References n n Needed for expressing location of points, lines, other objects Datums provide references in surveying n n Horizontally Vertically

Reference Ellipsoids Basic Concept a = semi-major axis b = semi-minor axis f = flattening e = eccentricity

Example Reference Ellipsoids Ellipsoid Clarke, 1866 GRS 80 WGS 84 Equatorial Axis Polar Axis Association 12, 756, 412. 8 m 12, 713, 167. 6 m NAD 27 datum 12, 756, 274 m 12, 713, 504. 6 m NAD 83 datum 12, 756, 274 m 12, 713, 504. 6 m GPS ITRS 12, 756, 272. 98 m 12, 713, 503. 5 m ITRF GRS = Geodetic Reference System WGS = World Geodetic System ITRS = International Terrestrial Reference System

Ignoring Earth Curvature n Distance 006 m 00. 80 ( 5 mi les+ 0. 2 5”) 8000. 000 m ( 5 miles) 1000 km 998. 95 km

Ignoring Earth Curvature (2) n Level line Horizontal plane 1 mile (1609 m) 8 inches ( 20 cm) Level surface

Ignoring Earth Curvature (3) n Triangle geometry mi 2 75 (48, 000 acres) 19, 800 hectares Sum of Interior Angles = 180° 00' 01"

Digitizing and Scanning n Instruments n Georeferencing n The process and problems associated with it n Automation n Formats

Why Do We Have To Digitize? n n n Existing data sets are general purpose, so if you want something specific you have to create it In spite of 20+ years of GIS, most stuff is still in analog form Chances are somebody else has digitized it before; but data sharing is not what it should be

Digitizer n n Digitizing table 10” x 10” to 80” x 60” $50 - $2, 000 1/100 th inch accuracy Stylus or puck with control buttons

The Digitizing Procedure n Affixing the map to the digitizer n Registering the map n Actual digitizing n n In point mode In stream mode

Georeferencing n n n at least 3 control points aka reference points or tics easily identifiable on the map exact coordinates need to be known

Digitizing Modes n Point mode n n n most common selective choice of points digitized requires judgment for man-made features Stream mode n n n large number of (redundant) points requires concentration For natural (irregular) features

Problems With Digitizing n Paper instability n Humidity-induced shrinking of 2%-3% n Cartographic distortion, aka displacement Overshoots, gaps, and spikes n Curve sampling n

Errors From Digitizing n n Fatigue Map complexity n ½ hour to 3 days for a single map sheet n Sliver polygons n Wrongly placed labels

Digitizing n Rule of thumb: one boundary per minute ergo: appr. 62 lines = more than one hour Costs

Automated Data Input (Scanning) n n Work like a photocopier or fax machine Three types: n Flatbed scanners n n Drum scanner n n n A 4 or A 3 600 to 2400 dpi optical resolution $50 to $2, 000 practically unlimited paper size $10 k TO $50 k Video line scanner n produces vector data

Requirements for Scanning n n n Data capture is fast but preparation is tedious Computers cannot distinguish smudges Lines should be at least 0. 1 of a mm wide Text and preferably color separation AI techniques don’t work (yet? ) Symbols such as are too variable for automatic detection and interpretation

Semi-automatic Data Input (Heads-up Digitizing) n Reasonable compromise between traditional digitizing and scanning n Much less tedious n Incorporating your intelligence

Criteria for Choosing Input Mode n n Images without easily detectable line work should be left in raster format Really dense line work should be left as background image – n unless it is really needed for automatic GIS analysis; in which case you would have to bite the bullet

Conversion from Other Databases n n n Autocad. dxf and d. BASE. dbf are de facto standards for GIS data exchange In the raster domain there is no equivalent; . tif comes closest to a “standard” In any case: merging data that originate from different scales is problematic – in the best of all worlds; there is no automatic generalization routine