Further by downloading this data the user acknowledges that they agree with the NERC data policy (), and the following conditions:

1. To cite the data in any publication as follows:

Jones, P. C. (2020). Processed line aerogravity data over the Evans Ice Stream, Southern Palmer Land region (1994/95 season) (Version 1.0) [Data set]. UK Polar Data Centre, Natural Environment Research Council, UK Research & Innovation. https://doi.org/10.5285/883474C4-CB40-4949-B7AF-EFCC1DAFC084

2. The user recognizes the limitations of data. Use of the data is at the users' own risk, and there is no warranty as to the quality or accuracy of any data, or the fitness of the data for your intended use. The data are not necessarily fully quality assured and cannot be expected to be free from measurement uncertainty, systematic biases, or errors of interpretation or analysis, and may include inaccuracies in error margins quoted with the data.

Methodology:

Airborne gravity data are presented in Jones et al (2002). Data are available on a flight by flight basis.

Processing steps:

1/ Calculate observed gravity.

True spring tension (ST_real) is calculated from the posted spring tension (ST) correcting for the fact that for this survey the true spring tension approaches the posted value at 38 mGal per second.

Beam velocity (Beam_vel) is derived from raw beam position (RB) assuming a centred difference approximation.

Relative gravity (rec_grav) = (Spr_tens_real+((beam_vel)*k_fac)+CC)*scale_value, k_fac=30, meter scale value =0.9966.

Still readings (Still) are in mGal and were calculating assuming a 2nd order best fit to the approximately linear drift of the meter observed at the tie down points.

Airborne absolute gravity values (Abs_grav) = Rec_grav- Still + Base

2/ Corrections to derive free air anomalies (disturbances).

Vertical acceleration (VaccCor) is calculated as 2nd derivative of flight altitude (Height_WGS1984), with a 3 point mean filter applied after differencing to reduce short wavelength noise.

Eotvos correction (EotvosCor) follows (Harlan, 1968).

Latitude correction (LatCor) = 978.03185(1+0.005278895 sin2Lat- 0.000023462 sin4Lat) (IUGG 1967).

Free air correction (FaCor) = 0.3086*Height_WGS1984. NOTE subsequent free air values are defined as gravity disturbances in geodesy, as they are referred to the ellipsoid (Hackney and Featherstone, 2003).

Horizontal acceleration correction (HaccCor). For this survey the approximation of (Swain, 1996) was used, assuming a damping factor of 0.707, and a platform period of 4 minutes.

3/ Free air anomaly and filtering.

Free air anomaly (Free_air) = Abs_grav-VaccCor+EotvosCor+FaCor-LatCor-(0.5*HaccCor)

Filtered free air anomaly (FAA_filt) used 9 km 1/2 wavelength space domain kernel filter (Holt et al., 2006).

Final free air data (FAA_clip) was produced by manually masking turns, start and end of lines, and other regions of noisy data.

Upward continued free air anomaly (Faacont) was produced by upward continuing free air data from the collected flight altitude to 2800 m.

Channel description:

Basic Channels

Date UTC date (YYYY/MM/DD)

Time UTC time (HH:MM:SS.SS)

FlightID Sequential flight number and survey ID e.g. W12

Line_name Line Number e.g. LW200.1:12

Lon Longitude WGS 1984, for processing see readme

Lat Latitude WGS 1984, for processing see readme

x x projected meters*

y y projected meters*

Height_WGS1984 Aircraft altitude (meters) in WGS 1984, for processing see location data page

Raw gravity Channels

ST Spring Tension (meter units)

CC Cross Coupling (meter units)

RB Raw beam position (Mv)

XACC Cross axis accelerometer (Mv)

LACC Long axis accelerometer (Mv)

Still Airborne meter still reading value (mGal)

Base Absolute gravity reference, from land gravity (mGal)

Calculation Channels

St_real True Spring tension value (meter units)

Beam_vel Gravity meter beam velocity (Mv/sec)

Rec_grav Recalculated relative gravity (mGal)

Abs_grav Calculated absolute gravity (mGal)

VaccCor Vertical acceleration correction

EotvosCor Eotvos correction

LatCor Latitude correction

FaCor Free air correction

HaccCor Horizontal acceleration correction

Free air Channels

FaaCont Free air anomaly data upward continued to an elevation of 2000 m

*Projected coordinates (x and y) are in Polar sterographic defined as follows:

Latitude of natural origin: -71

Longitude of natural origin: 0

Scale factor at natural origin: 0.994

False easting 0

False northing 2082760.109

Positioning for the Evans ice stream survey uses kinematic differential GPS. Processing was carried out using GPSurvey® software. Differential positioning used fixed base stations at each of the ...(3)

Data quality:

No values are given for Still, Base, St_Real, Beam_vel, Rec_grav, Abs_grav, VaccCor, EotvosCor, LatCor, FaCor, HaccCor. Intermediate information not available for this legacy data.