Airborne Gravity: NGS' Gravity Data for PN03 (2017-2018)

Airborne gravity data for Idaho, Montana, Washington, and Canada collected 2017-2018 over 4 surveys. This data set is part of the Gravity for the Re-definition of the American Vertical Datum (GRAV-D) project initiated by NOAA's National Geodetic Survey to collect and monitor gravity data suitable for the re-definition of the vertical datum for at least the United States and territories. The data is available at http://www.ngs.noaa.gov/GRAV-D/data_pn03.shtml in ASCII text format.

These data sets were collected and processed by NOAA's National Geodetic Survey (www.ngs.noaa.gov) as part of its GRAV-D effort to modernize the U.S. vertical datum. The purpose of these data is to provide accurate gravity data for geoid modeling for the U.S. and its territories. It was designed to provide medium wavelength gravity information (around 20-400 km) to connect satellite and ground gravity measurements.

The ASCII text file provides a single processed observation per row with the following elements comma separated: year, month, day, hour, minutes, seconds, latitude (dd), longitude (dd), ellipsoid height (m), full field gravity at altitude (mGals), full field EGM08 gravity at altitude (mGal)

http://www.ngs.noaa.gov/GRAV-D/data_pn03.shtml

While every effort has been made to ensure that these data are accurate and reliable within the limits of the current state of the art, NOAA cannot assume liability for any damages caused by any errors or omissions in the data, nor as a result of the failure of the data to function on a particular system. NOAA makes no warranty, expressed or implied, nor does the fact of distribution constitute such a warranty.

https://www.ngs.noaa.gov/grav-d/data_products.shtml

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Must read and fully comprehend the metadata prior to use, acknowledgment of the Originator when using the data set as a source, data should not be used beyond the limits of the source scale

The absolute accuracy of airborne gravity is difficult to quantify. The following measures are indicators of data quality, but are not definitive measures. The cross over precision is 4.00 mGals, which is a combination of differences between cross track and data track and may not be indicative of data quality. Adjacent data lines have an average cross-correlation of 99.59%. Low correlation between adjacent lines may indicate either poor quality on one of the two lines or rapidly changing features. More detailed information can be found at http://www.ngs.noaa.gov/GRAV-D/data_pn03.shtml.

Due to the altitude, speed of data collection, and track spacing of 10 kilometers the resolution of these data are limited to 20 kilometers. More detailed information can be found at http://www.ngs.noaa.gov/GRAV-D/data_pn03.shtml.

Data processing is fully automated and output file is computer generated. All lines are checked by an analyst and any errors in production are corrected. More detailed information can be found at http://www.ngs.noaa.gov/GRAV-D/data_pn03.shtml.

Data are processed shortly after flying to ensure that all instruments are working and data quality is acceptable. Field personnel perform data checks at least once per day to ensure all GPS receivers, the gravimeter, and IMUs are working up to standard. The field personnel also process GPS and gravity data, mapping the preliminary gravity product, to check quality. Additional quality control plots are built into the GPS and gravity processing software packages for advanced data analysis in the field.

After processing data are compared to an existing geoid model. In addition processed gravity is compared at common points from several flight lines crossing the survey lines to verify consistent gravity measurements in the same location.. More detailed information can be found at http://www.ngs.noaa.gov/GRAV-D/data_pn03.shtml.