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| Titel |
Measurement of surface moisture using infra-red terrestrial laser scanning |
| VerfasserIn |
Gerben Ruessink, Laura Brakenhoff, Marcel van Maarseveen |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250088660
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| Publikation (Nr.) |
 EGU/EGU2014-2797.pdf |
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| Zusammenfassung |
| The measured aeolian supply of sand from the beach to the foredune is generally less than the
potential supply because the wetness of the beach surface limits sand entrainment and supply.
The strong spatial and temporal variability in surface moisture is, however, notoriously
difficult to determine, which has prevented the development of quantitatively more realistic
sand-supply models. Here we test the possibility of deriving surface moisture content from
the reflectance signal of an infra-red terrestrial laser scanner (TLS) in a large area
(~ 100 x 100m) with high spatial (~ 0.25 x 0.25m) and temporal (~ 30 minutes)
resolution.
We deployed a RIEGL VZ-400 3D laser scanner (short-wave infra-red, wavelength =
1550 nm) from a tripod at Egmond Beach, The Netherlands, with the instrument about
2m above beach level. The TLS collected data through 360° in the horizontal
plane and 100° in the vertical, with a rate of 122,000points/s. A full 360°/100°
(panorama) scan with a 0.02° resolution in the horizontal and vertical took about
12minutes to complete. Nine panorama scans were performed over a single tidal
cycle, during which also 69surface scrapings (thickness of a few millimetres) were
taken. After each panorama scan, a cross-shore profile from the dunefoot to the
shoreline was scanned 10 times per second for 2 minutes to explore the repeatability
of the TLS data. The nine scans were processed into 0.25 x 0.25m reflectance
maps, while the scrapings were processed into moisture estimates using standard
laboratory techniques. The RIEGL VZ-400 expresses reflectance in decibels, with
a 1-R2 correction to account for the reduction of returned intensity with range
R.
We found that reflectance does not change for R = 15 - 60m over beach sand with
spatially invariant moisture content. Apparently, the returned reflectance is then indeed
proportional to 1-R2 and independent of the angle between the laser pulse and the beach
surface. For R < 15m, the 1-R2 relationship does not hold, while for R > 60m
reflectance drops off rapidly, most likely due to the strong distortion of the laser
footprint at these range values. We also observed that reflectance is linearly related
to surface moisture content for the full range from dry to saturated sand, with a
standard error of about 2.6%. This represents an improvement over earlier TLS studies
with smaller wavelengths, in which reflectance was insensitive to moisture content
larger than some 5%. Also, the standard error is lower than typically reported for
optical (video) estimates of moisture content. Finally, we found that the reflectance
measurements are highly robust, with a typical repeatability in moisture content of about
0.3%. To conclude, short-wave infra-red TLS has the capability to derive values of
surface moisture accurately and robustly over its full range in a spatially extensive
area at spatial and temporal resolution infeasible with standard in-situ techniques. |
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