 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Geodetic control network" – news ·newspapers ·books ·scholar ·JSTOR(August 2016) (Learn how and when to remove this message) |
Ageodetic control network is a network, often oftriangles, that are measured precisely by techniques ofcontrol surveying, such as terrestrialsurveying orsatellite geodesy. It is also known as ageodetic network,reference network,control point network, or simplycontrol network.
A geodetic control network consists ofgeodetic markers, which are stable, identifiable points or vertices with published coordinate values derived from observations that tie the points together.[1]
In the U.S., there is a national control network called theNational Spatial Reference System (NSRS). Many organizations may contribute information to the geodetic control network.[2][3] In the United Kingdom, theOrdnance Survey maintains theOS Net network.[4]
The higher-order (high precision, usually millimeter-to-decimeter on a scale of continents) control points are normally defined in both space and time using global or space techniques, and are used for "lower-order" points to be tied into. The lower-order control points are normally used forengineering,construction andnavigation. The scientific discipline that deals with the establishing of coordinates of points in a control network is calledgeodesy.
After a cartographer registers key points in a digital map to the real world coordinates of those points on the ground, the map is then said to be "in control". Having a base map and other data in geodetic control means that they will overlay correctly.
When map layers are not in control, it requires extra work to adjust them to line up, which introduces additional error.Those real world coordinates are generally in some particularmap projection, unit, andgeodetic datum.[5]
Survey control networks are used for different phases of engineering and construction. Applications, according to theChartered Institution of Civil Engineering Surveyors, include:[6]
An example application is the accurate and robust positioning oftunnel boring machines.[7]
In "classical geodesy" (up to the sixties) control networks were established bytriangulation using measurements ofangles and of some spare distances. The precise orientation to thegeographic north is achieved through methods ofgeodetic astronomy. The principal instruments used aretheodolites andtacheometers, which nowadays are equipped withinfrared distance measuring,data bases, communication systems and partly by satellite links.
Electronic distance measurement (EDM) was introduced around 1960, when theprototype instruments became small enough to be used in the field. Instead of using only sparse and much less accurate distance measurements some control networks were established or updated by usingtrilateration more accurate distance measurements than was previously possible and no angle measurements.
EDM increased networkaccuracies up to 1:1 million (1 cm per 10 km; today at least 10 times better), and made surveying less costly.
Optical levelling, which determines relative height difference between two points, can be used to determine the vertical component of control networks, particularly relative to fixed datums (such as that employed forOrdnance Datum Newlyn). Double run level techniques may be employed to minimise error.[8][9]
The geodetic use ofsatellites began around the same time. By using bright satellites likeEcho I,Echo II andPageos, global networks were determined, which later provided support for the theory ofplate tectonics.
Another important improvement was the introduction ofradio and electronic satellites likeGeos A and B (1965–70), of theTransit system (Doppler effect) 1967-1990 — which was the predecessor of GPS - and oflaser techniques likeLAGEOS (USA, Italy) orStarlette (France). Despite the use of spacecraft, small networks forcadastral andtechnical projects are mainly measured terrestrially, but in many cases incorporated in national and global networks by satellite geodesy.
Nowadays, several hundred geospatial satellites are in orbit, including a large number ofremote sensing satellites andnavigation systems likeGPS andGlonass, which was followed by the EuropeanGalileo satellites in 2020 and China'sBeidouconstellation.
While these developments have made satellite-based geodetic network surveying more flexible and cost effective than its terrestrial equivalent for areas free of tree canopy or urban canyons, the continued existence offixed point networks is still needed for administrative and legal purposes on local and regional scales. Global geodetic networks cannot be defined to be fixed, sincegeodynamics are continuously changing the position of allcontinents by 2 to 20 cm per year. Therefore, modern global networks likeETRS89 orITRF show not onlycoordinates of their "fixed points", but also their annualvelocities.
