Transformation of Coordinates among Official Referential Systems - ProGriD
ProGriD is a Windows application designed to convert coordinates among the official reference systems used in Brazil: Córrego Alegre, SAD69 and SIRGAS2000. It is the result of an effort to provide the users community of geospatial data with a tool that streamlines the transition to SIRGAS2000.
ProGriD represents a breakthrough in the conversion of coordinates among the geodetic reference frames. In general, two geodetic reference frames relate with each other through constant conversion parameters for any area covered by such frames. What cannot be converted by these parameters become remainders, representing the distortions of the geodetic network. ProGriD models these remainders – i.e, the network distortion itself –, providing a more precise result.
ProGriD uses files containing a grid of values in latitude and longitude, which allows the direct conversion among Córrego Alegre, SAD69 and SIRGAS2000, according to the NTv2 format. The selection of the NTv2 grid was due to the fact that many SIG and GPS computer programs are able to read this format.
ProGriD converts the coordinates from the following materializations:
- The materialization of Córrego Alegre in 1961, called Córrego Alegre (1961) in ProGriD.
- The materializations of Córrego Alegre in 1970 and 1972, treated together and called Córrego Alegre (1970+1972).
- The original materialization of SAD69, just including the classic network, simply called SAD69 Classic Network.
- The materialization of SAD69 in 1996, just including the classic network, called SAD69/96 Classic Network.
- SAD69 Doppler Technique or GPS.
It should be stressed that in the conversion of coordinates between the "SAD69 Doppler Technique or GPS" and "SIRGAS2000" – and vice-versa –, ProGriD uses the parameters described in the IBGE´s Presidency Resolution no. 1, of February 25, 2005 (R.PR 01/05) and, thus, produces the same results obtained by the TCGEO program.
These parameters were estimated based on a set of 63 geodetic stations from the GPS Network of the Brazilian Geodetic System - SGB. Considering that the GPS Network of the SGB has a distortion pattern much smaller than those established by classic methods – triangulation and polygonation –, the modeling of distortions is not needed, since they are homogeneous and small. These options also accept as input and output the format of Cartesian coordinates, which can be selected by users when input coordinates are obtained through GPS or Doppler surveying.
The coordinates of the Doppler stations of the SGB were established through the point positioning method, i.e., these coordinates were obtained independently from the classic and GPS networks, not requiring the modeling of distortions.
The coordinate types associated with each referential are:
- Córrego Alegre (1961): latitude / longitude and UTM (E, N).
- Córrego Alegre (1970+1972): latitude / longitude and UTM (E, N).
- SAD69 Classic Network: latitude / longitude and UTM (E, N).
- SAD69/96 Classic Network: latitude / longitude and UTM (E, N).
- SAD69 Doppler/GPS Technique: latitude / longitude / geometric height, Cartesian (X, Y, Z) and UTM (E, N).
- SIRGAS2000: latitude / longitude / geometric height, Cartesian (X, Y, Z) and UTM (E, N).
Many features used in ProGriD came from user suggestions through scientific events and an online questionnaire.