## IBGE releases new values of reference heights

July 30, 2018 10h00 AM | Last Updated: July 30, 2018 02h41 PM

The IBGE releases today the calculation of new high-precision heights, which serve as a basis for engineering, mapping and scientific studies all over the national territory. The 2018 Report on the Readjustment of the Altimetric Network with Geo-potential Numbers is available here.

Called "Readjustment of the Altimetric Network with Geo-potential Numbers" - REALT-2018, the study of the heights along the Brazilian territory aims at developing and implementing a new methodology for the complete revision of the databases and the unprecedented integration of information of the gravity variation. As a result, Brazil becomes in line with the international recommendations to establish, at a short term, an International Height Reference System, whose first version will include six reference stations in Brazil.

Since it started to measure the geometric leveling in 1945, the IBGE periodically recalculates the heights of the Level References - RRNN of the High-Precision Altimetric Network - RAAP of the Brazilian Geodetic System - SGB through the so-called "adjustment by the least squares method". Those calculations incorporate new observations of geometric leveling and gravimetry, as well as the correction of inconsistencies casually detected in the RAAP and the use of new techniques of observation and calculation.

The integration of the gravity values into the RRNN allows to obtain more precise heights, since that gravity in the terrestrial surface mainly varies as we move away from the equatorial line towards the poles by between 9,764 m/s2 and 9,834 m/s2. Therefore, the inclusion of the gravity avoids altimetric inconsistencies, helping both the integration of the height network between neighbor countries and the study of the impacts of the rising of the average sea level at the coastal zone.

**New height type**

The physical heights and the geo-potential numbers that serve them as a basis are the appropriate coordinates for use in the vertical positioning, since they toughly link with the gravity field. On the other hand, the so-called geometric or ellipsoidal heights, resulting from the modern techniques of the Spatial Geodesy – like the Global Navigation Satellite Systems - GNSS –, do not keep any link with the gravity field and, thus, are not appropriate for use in the vertical positioning.

Due to the lack of sufficient gravimetric observations, only the theoretical correction of gravity was applied to the observed gaps in the previous adjustments (2011, 1993, 1975~1959), resulting in simplified orthometric heights. It was possible to obtain real values of gravity for all the RRNN of the RAAP in REALT-2018 and, thus, calculate the so-called normal heights, more appropriate to the modern concepts and methods of Geodesy.

The decision to carry out a new adjustment, started in 2015, was due to the need to update the vertical component of the SGB, in accord with the resolution of the International Association of Geodesy - IAG on the International Height Reference System - IHRS/IHRF. In addition, the huge improvement of the gravimetric coverage of the Brazilian territory carried out by the IBGE and other institutions over the last decades was also key to that. As result, the SGB users will have heights consistent with the global geodetic models for the first time ever.