INTRODUCTION - Field Geophysics - page 13

Google Earth is familiar not only to geophysicists but to almost everyone
who has internet access. Only freely available satellite imagery and aerial
photographs are used, and quality and geo-registration accuracy vary with
location. Images can be saved as .jpg files and it is possible, before so doing,
to superimpose survey area outlines or survey grids using standard .kml
(ASCII) or .kmz (binary) files. Area dimensions can be quickly estimated,
and the area to be surveyed (and the parts that may be unsurveyable because
of access restrictions) can be discussed and agreed with the client. The im-
ages also provide a practical basis for planning access along routes through
farmers' fields. 'Forewarned, forearmed; to be prepared is half the victory'
[Miguel de Cervantes Saavedra, seventeenth-century Spanish writer].
Internet-available elevation grids are less widely known, but can be
equally useful. The Satellite Radar Tracking Mission (SRTM) used a sate-
llite-mounted synthetic aperture radar interferometer to obtain data during
a period of 11 days in February 2000. The targeted landmass extended from
56 ◦ Sto60 ◦ N, and within this region (containing about 80% of the Earth's
land surface) elevation estimates were obtained once for at least 99.96%,
twice for at least 94.59% and three or more times for about 50%. The data
are now available as one-degree square 'tiles', with a 3 arc-second cell-size
(equivalent to about 90 m at the Equator) globally (SRTM3) and an op-
tional 1 arc-second (30-m) cell size in the USA (SRTM1). The Version 2
processed data set was replaced in 2009 by an improved (although usually
imperceptibly so) Version 2.1.
The SRTM data as distributed suffered from data gaps in areas of steep
topographic gradients. It was inevitable that, with a swathe width of about
225 km and a satellite altitude of 233 km, there would be areas that could
not be imaged by a side-looking system. These disadvantages have, to a
considerable extent, been overcome in the ASTER (Advanced Spaceborne
Thermal Emission and Reflection Radiometer) data obtained with Japanese
instrumentation mounted from December 1999 onwards on a US Terra
spacecraft. Coverage was also wider, from 83 ◦ Sto83 ◦ N. As with SRTM,
ASTER data are distributed in one-degree 'tiles' but with a worldwide 1 arc-
second ( ∼ 30-m) cell size. Elevation data are provided in GeoTIFF format,
and each data file is accompanied by a quality (QA) file that indicates data
reliability, pixel by pixel.
The ASTER instrument operated stereoscopically in the near infra-red,
and could therefore be affected by cloud cover. In most cases this problem
was solved by the high degree of redundancy (since the mission lasted much
longer than the 11 days of SRTM), but in some cases SRTM data have had
to be used for infill. In a few areas where SRTM coverage did not exist,
'bad' pixels remain and are flagged by a − 9999 value.