Corrections
Corrections applied to the observations are as follows, subject to available data. For example, no rod or level corrections can be applied if calibration data are not in the dababase and the level’s collimation error has not been determined.
- Orthometric (usually accounted for by using geopotential number differences, rather than as a “correction”; see below for additional information)
- Rod Scale
- Rod Temperature
- Level Collimation (applied in the digital level instrument)
- Refraction
- Astronomic
- Magnetic (currently only applicable to Zeiss NI-1 levels)
More information on corrections 1-6 above may be found in NOAA Technical Memorandum NOS NGS 34,“Corrections Applied By The National Geodetic Survey To Precise Leveling Observations”. The orthometric correction referred to in NOS NGS 34 was used for NGVD 29 but not for NAVD 88, and it is not based on observed gravity. It is important to note that there are other versions of the orthometric correction based on observed gravity, but these are not given in NOS NGS 34. The “Height Systems” section at the end of NOS NGS 34 includes a discussion of orthometric corrections, gravity, geopotential numbers, and other geodetic issues of height determination. More information on correction 7 above may be found in NOAA Technical Memorandum NOS NGS-45, “Empirical calibration of Zeiss NI-1 level instruments to account for magnetic errors” by Holdahl, Sandford R., June 1986 (not yet available online).
For NAVD 88, the observed (leveled) height differences are combined with surface gravity to compute geopotential number differences, rather than by applying orthometric corrections to leveled height differences. The geopotential numbers are adjusted and then converted to orthometric heights. This approach reduces the systematic error in the same manner as orthometric corrections, and it also allows determination of other types of heights from the same observations, such as dynamic heights.
Whether geopotential numbers or orthometric corrections are used, the intent is the same: to reduce systematic error due to non-parallelism of level surfaces. This error can be substantial, especially in high elevation areas and mountainous terrain. The LVL_DH - Leveled Height Difference Computation tool in the NGS Geodetic Toolkit provides a means to assess the magnitude of this error, by converting orthometric heights differences to leveled height differences (in effect removing the orthometric correction). This tool also includes documentation providing further information on the topic.
Additional information on the geodesy of leveling (including corrections) is described in the PowerPoint presentation “4_Geodesy_and_Corrections_for_Leveling_v2.ppt” available from the NGS LOCUS Reviewed site. This site also includes several other presentations used for NGS workshops on geodetic leveling. Definitions of geopotential numbers, orthometric heights, dynamic heights, and other terms related to the geodesy of heights are available in the NGS Geodetic Glossary.
