Editing Geographic information system (section)

===Data acquisition===
[[File:Field-Map birdie.jpg|thumb|right|Example of hardware for mapping ([[GPS]] and [[laser rangefinder]]) and data collection ([[rugged computer]]). The current trend for geographical information system (GIS) is that accurate mapping and data analysis are completed while in the field. Depicted hardware ([[field-map]] technology) is used mainly for [[forest inventory|forest inventories]], monitoring and mapping.]]
GIS data acquisition includes several methods for gathering spatial data into a GIS database, which can be grouped into three categories: ''primary data capture'', the direct measurement phenomena in the field (e.g., [[remote sensing]], the [[global positioning system]]); ''secondary data capture'', the extraction of information from existing sources that are not in a GIS form, such as paper maps, through [[digitization]]; and ''[[List of GIS data sources|data transfer]]'', the copying of existing GIS data from external sources such as government agencies and private companies. All of these methods can consume significant time, finances, and other resources.<ref name="longley2015"/>{{rp|page=173}}

====Primary data capture====
[[Surveying|Survey]] data can be directly entered into a GIS from digital data collection systems on survey instruments using a technique called [[coordinate geometry]] (COGO). Positions from a global navigation satellite system ([[Satellite navigation|GNSS]]) like the [[Global Positioning System]] can also be collected and then imported into a GIS. A current trend{{As of?|date=September 2024}} in data collection gives users the ability to utilize [[Rugged computer|field computers]] with the ability to edit live data using wireless connections or disconnected editing sessions.<ref>{{cite journal|last1=Marwick|first1=Ben|last2=Hiscock|first2=Peter|last3=Sullivan|first3=Marjorie|last4=Hughes|first4=Philip|title=Landform boundary effects on Holocene forager landscape use in arid South Australia|journal=Journal of Archaeological Science: Reports|volume=19|pages=864–874|date=July 2017|doi=10.1016/j.jasrep.2017.07.004|s2cid=134572456}}</ref> The current trend{{As of?|date=September 2024}} is to utilize applications available on smartphones and [[Personal digital assistant|PDAs]] in the form of mobile GIS.<ref>{{citation |last1=Buławka |first1=Nazarij |last2=Chyla |first2=Julia Maria |chapter=Mobile GIS in Archaeology – Current Possibilities, Future Needs. Position Paper |chapter-url=https://publikationen.uni-tuebingen.de/xmlui/handle/10900/101847 |title=CAA: Digital Archaeologies, Material Worlds (Past and Present) |year=2020 |publisher=Tübingen University Press |location=Tübingen |isbn=978-3-947-25115-5 |s2cid=246410784}}</ref> This has been enhanced by the availability of low-cost mapping-grade GPS units with decimeter accuracy in real time. This eliminates the need to post process, import, and update the data in the office after fieldwork has been collected. This includes the ability to incorporate positions collected using a [[laser rangefinder]]. New technologies also allow users to create maps as well as analysis directly in the field, making projects more efficient and mapping more accurate.

[[Remote sensing|Remotely sensed]] data also plays an important role in data collection and consist of sensors attached to a platform. Sensors include cameras, digital scanners and [[lidar]], while platforms usually consist of aircraft and [[satellite]]s. In England in the mid-1990s, hybrid kite/balloons called [[Allsopp Helikite|helikites]] first pioneered the use of compact airborne digital cameras as airborne geo-information systems. Aircraft measurement software, accurate to 0.4&nbsp;mm, was used to link the photographs and measure the ground. Helikites are inexpensive and gather more accurate data than aircraft. Helikites can be used over roads, railways and towns where [[unmanned aerial vehicle]]s (UAVs) are banned.

Recently, aerial data collection has become more accessible with [[miniature UAV]]s and drones. For example, the [[Aeryon Scout]] was used to map a 50-acre&nbsp;area with a [[ground sample distance]] of {{convert|1|in|cm|2}} in only 12&nbsp;minutes.<ref>{{cite web |url=http://www.aeryon.com/news/pressreleases/248-softwareversion5.html |title=Aeryon Announces Version 5 of the Aeryon Scout System &#124; Aeryon Labs Inc |publisher=Aeryon.com |date=6 July 2011 |access-date=13 May 2012 |archive-date=10 June 2020 |archive-url=https://web.archive.org/web/20200610142031/http://www.aeryon.com/news/pressreleases/248-softwareversion5.html |url-status=dead }}</ref>

The majority of digital data currently comes from [[photo interpretation]] of aerial photographs. Soft-copy workstations are used to digitize features directly from [[Stereoscopy|stereo pairs]] of digital photographs. These systems allow data to be captured in two and three dimensions, with elevations measured directly from a stereo pair using principles of [[photogrammetry]]. Analog aerial photos must be scanned before being entered into a soft-copy system, for high-quality digital cameras this step is skipped.

Satellite [[remote sensing]] provides another important source of spatial data. Here satellites use different sensor packages to passively measure the reflectance from parts of the [[electromagnetic spectrum]] or radio waves that were sent out from an active sensor such as radar. Remote sensing collects raster data that can be further processed using different bands to identify objects and classes of interest, such as land cover.

====Secondary data capture====
{{further|Digitizing}}
The most common method of data creation is [[Digitizing|digitization]], where a [[hard copy]] map or survey plan is transferred into a digital medium through the use of a CAD program, and geo-referencing capabilities. With the wide availability of [[Orthophoto|ortho-rectified imagery]] (from satellites, aircraft, Helikites and UAVs), heads-up digitizing is becoming the main avenue through which geographic data is extracted. Heads-up digitizing involves the tracing of geographic data directly on top of the aerial imagery instead of by the traditional method of tracing the geographic form on a separate [[Graphics tablet|digitizing tablet]] (heads-down digitizing). Heads-down digitizing, or manual digitizing, uses a special magnetic pen, or stylus, that feeds information into a computer to create an identical, digital map. Some tablets use a mouse-like tool, called a puck, instead of a stylus.<ref>{{Cite journal|last=Puotinen|first=Marji|date=June 2009|title=A Primer of GIS: Fundamental Geographic and Cartographic Concepts - By Francis Harvey|journal=Geographical Research|volume=47|issue=2|pages=219–221|doi=10.1111/j.1745-5871.2009.00577.x|bibcode=2009GeoRs..47..219P |issn=1745-5863|doi-access=free}}</ref><ref name=":2">{{Cite web|title=Digitizing - GIS Wiki {{!}} The GIS Encyclopedia|url=http://wiki.gis.com/wiki/index.php/Digitizing|access-date=2021-01-29|website=wiki.gis.com}}</ref> The puck has a small window with cross-hairs which allows for greater precision and pinpointing map features. Though heads-up digitizing is more commonly used, heads-down digitizing is still useful for digitizing maps of poor quality.<ref name=":2" />

Existing data printed on paper or [[PET film (biaxially oriented)|PET film]] maps can be [[digitizer|digitized]] or scanned to produce digital data. A digitizer produces [[Vector graphics|vector]] data as an operator traces points, lines, and polygon boundaries from a map. [[Image scanner|Scanning]] a map results in raster data that could be further processed to produce vector data.

When data is captured, the user should consider if the data should be captured with either a relative accuracy or absolute accuracy, since this could not only influence how information will be interpreted but also the cost of data capture.

After entering data into a GIS, the data usually requires editing, to remove errors, or further processing. For vector data it must be made "topologically correct" before it can be used for some advanced analysis. For example, in a road network, lines must connect with nodes at an intersection. Errors such as undershoots and overshoots must also be removed. For scanned maps, blemishes on the source map may need to be removed from the resulting [[Raster graphics|raster]]. For example, a fleck of dirt might connect two lines that should not be connected.