GIS techniques and technology Geographic information system

1 gis techniques , technology

1.1 relating information different sources
1.2 gis uncertainties
1.3 data representation
1.4 data capture
1.5 raster-to-vector translation
1.6 projections, coordinate systems, , registration

gis techniques , technology

modern gis technologies use digital information, various digitized data creation methods used. common method of data creation digitization, hard copy map or survey plan transferred digital medium through use of cad program, , geo-referencing capabilities. wide availability of ortho-rectified imagery (from satellites, aircraft, helikites , uavs), heads-up digitizing becoming main avenue through geographic data extracted. heads-up digitizing involves tracing of geographic data directly on top of aerial imagery instead of traditional method of tracing geographic form on separate digitizing tablet (heads-down digitizing).

relating information different sources

gis uses spatio-temporal (space-time) location key index variable other information. relational database containing text or numbers can relate many different tables using common key index variables, gis can relate otherwise unrelated information using location key index variable. key location and/or extent in space-time.

any variable can located spatially, , increasingly temporally, can referenced using gis. locations or extents in earth space–time may recorded dates/times of occurrence, , x, y, , z coordinates representing, longitude, latitude, , elevation, respectively. these gis coordinates may represent other quantified systems of temporo-spatial reference (for example, film frame number, stream gage station, highway mile-marker, surveyor benchmark, building address, street intersection, entrance gate, water depth sounding, pos or cad drawing origin/units). units applied recorded temporal-spatial data can vary (even when using same data, see map projections), earth-based spatial–temporal location , extent references should, ideally, relatable 1 , real physical location or extent in space–time.

related accurate spatial information, incredible variety of real-world , projected past or future data can analyzed, interpreted , represented. key characteristic of gis has begun open new avenues of scientific inquiry behaviors , patterns of real-world information had not been systematically correlated.

gis uncertainties

gis accuracy depends upon source data, , how encoded data referenced. land surveyors have been able provide high level of positional accuracy utilizing gps-derived positions. high-resolution digital terrain , aerial imagery, powerful computers , web technology changing quality, utility, , expectations of gis serve society on grand scale, nevertheless there other source data affect overall gis accuracy paper maps, though these may of limited use in achieving desired accuracy.

in developing digital topographic database gis, topographical maps main source, , aerial photography , satellite imagery sources collecting data , identifying attributes can mapped in layers on location facsimile of scale. scale of map , geographical rendering area representation type important aspects since information content depends on scale set , resulting locatability of map s representations. in order digitize map, map has checked within theoretical dimensions, scanned raster format, , resulting raster data has given theoretical dimension rubber sheeting/warping technology process.

a quantitative analysis of maps brings accuracy issues focus. electronic , other equipment used make measurements gis far more precise machines of conventional map analysis. geographical data inherently inaccurate, , these inaccuracies propagate through gis operations in ways difficult predict.

data representation

gis data represents real objects (such roads, land use, elevation, trees, waterways, etc.) digital data determining mix. real objects can divided 2 abstractions: discrete objects (e.g., house) , continuous fields (such rainfall amount, or elevations). traditionally, there 2 broad methods used store data in gis both kinds of abstractions mapping references: raster images , vector. points, lines, , polygons stuff of mapped location attribute references. new hybrid method of storing data of identifying point clouds, combine three-dimensional points rgb information @ each point, returning 3d color image . gis thematic maps becoming more , more realistically visually descriptive of set out show or determine.

for list of popular gis file formats, such shapefiles, see gis file formats § popular gis file formats.

data capture

example of hardware mapping (gps , laser rangefinder) , data collection (rugged computer). current trend geographical information system (gis) accurate mapping , data analysis completed while in field. depicted hardware (field-map technology) used forest inventories, monitoring , mapping.

data capture—entering information system—consumes of time of gis practitioners. there variety of methods used enter data gis stored in digital format.

existing data printed on paper or pet film maps can digitized or scanned produce digital data. digitizer produces vector data operator traces points, lines, , polygon boundaries map. scanning map results in raster data further processed produce vector data.

survey data can directly entered gis digital data collection systems on survey instruments using technique called coordinate geometry (cogo). positions global navigation satellite system (gnss) global positioning system can collected , imported gis. current trend in data collection gives users ability utilize field computers ability edit live data using wireless connections or disconnected editing sessions. has been enhanced availability of low-cost mapping-grade gps units decimeter accuracy in real time. eliminates need post process, import, , update data in office after fieldwork has been collected. includes ability incorporate positions collected using laser rangefinder. new technologies allow users create maps analysis directly in field, making projects more efficient , mapping more accurate.

remotely sensed data plays important role in data collection , consist of sensors attached platform. sensors include cameras, digital scanners , lidar, while platforms consist of aircraft , satellites. in england in mid 1990s, hybrid kite/balloons called helikites first pioneered use of compact airborne digital cameras airborne geo-information systems. aircraft measurement software, accurate 0.4 mm used link photographs , measure ground. helikites inexpensive , gather more accurate data aircraft. helikites can used on roads, railways , towns unmanned aerial vehicles (uavs) banned.

recently aerial data collection becoming possible miniature uavs. example, aeryon scout used map 50-acre area ground sample distance of 1 inch (2.54 cm) in 12 minutes.

the majority of digital data comes photo interpretation of aerial photographs. soft-copy workstations used digitize features directly stereo pairs of digital photographs. these systems allow data captured in 2 , 3 dimensions, elevations measured directly stereo pair using principles of photogrammetry. analog aerial photos must scanned before being entered soft-copy system, high-quality digital cameras step skipped.

satellite remote sensing provides important source of spatial data. here satellites use different sensor packages passively measure reflectance parts of electromagnetic spectrum or radio waves sent out active sensor such radar. remote sensing collects raster data can further processed using different bands identify objects , classes of interest, such land cover.

when data captured, user should consider if data should captured either relative accuracy or absolute accuracy, since not influence how information interpreted cost of data capture.

after entering data gis, data requires editing, remove errors, or further processing. vector data must made topologically correct before can used advanced analysis. example, in road network, lines must connect nodes @ intersection. errors such undershoots , overshoots must removed. scanned maps, blemishes on source map may need removed resulting raster. example, fleck of dirt might connect 2 lines should not connected.

raster-to-vector translation

data restructuring can performed gis convert data different formats. example, gis may used convert satellite image map vector structure generating lines around cells same classification, while determining cell spatial relationships, such adjacency or inclusion.

more advanced data processing can occur image processing, technique developed in late 1960s nasa , private sector provide contrast enhancement, false color rendering , variety of other techniques including use of 2 dimensional fourier transforms. since digital data collected , stored in various ways, 2 data sources may not entirely compatible. gis must able convert geographic data 1 structure another. in doing, implicit assumptions behind different ontologies , classifications require analysis. object ontologies have gained increasing prominence consequence of object-oriented programming , sustained work barry smith , co-workers.

projections, coordinate systems, , registration

the earth can represented various models, each of may provide different set of coordinates (e.g., latitude, longitude, elevation) given point on earth s surface. simplest model assume earth perfect sphere. more measurements of earth have accumulated, models of earth have become more sophisticated , more accurate. in fact, there models called datums apply different areas of earth provide increased accuracy, nad83 u.s. measurements, , world geodetic system worldwide measurements.