Peer-Reviewed Articles
1029 Sensitivity of
Digital Landscapes to Artifact Depressions in Remotely-Sensed DEMs
John B. Lindsay and Irena F. Creed
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Depressions are often removed from digital elevation models
(DEMs) used in hydro-geomorphic applications. Light detection
and ranging (lidar) and interferometric synthetic aperture
radar (INSAR) DEMs of flat to mountainous landscapes
were used to evaluate the occurrence of artifact depressions
caused by the representation of surfaces using grids and
random elevation error. The number of depressions in DEMs
that result from grid representation was inversely related to
grid spacing; however, normalizing for the number of grid
cells in a DEM demonstrated that coarser grids were relatively
more vulnerable to depressions. Flat landscapes containing
extensive lakes experienced more depressions related to grid
spacing and placement than high-relief areas. Stochastic
modelling showed that error magnitude controlled the extent
of vulnerability within a landscape to depressions caused
by random error. Nevertheless, certain areas were likely
to experience depressions regardless of the magnitude of
random error, including flat areas, valley bottoms, and highly
convergent topography.
1037 Image Misregistration
Error in Change Measurements
Hongqing Wang and Erle C. Ellis
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Planimetric positional error limits the accuracy of landscape
change measurements based on features interpreted from
high spatial resolution imagery (1 m), and this limitation
depends on the magnitude of the positional error, the
spatial heterogeneity of landscapes, and the spatial extent
of the change detection window (the change detection
resolution). For this reason, accuracy assessments of change
measurements from feature-based approaches require careful
evaluation of the impacts of positional errors across landscapes
differing in spatial heterogeneity at different change
detection resolutions. We quantified such impacts by
computing the false changes produced by spatially shifting
and comparing high-resolution ecological maps derived by
feature interpretation and ground interpretation of 1 m
resolution Ikonos imagery of rural China and 0.3 m resolution
aerial photographs of suburban United States. Change
detection error increased significantly as positional errors
increased, as landscape heterogeneity increased, and as the
change detection resolution became finer. Regression-derived
relationships between change estimation error and positional
error, change detection resolution, and landscape heterogeneity
allow calculation of the minimum change detection
window size at which it is possible to obtain change measurements
of a specified accuracy given any set of featurebased
ecological maps and their positional error. Prediction
of this “optimal change detection resolution” is critical
in
producing reliable high-resolution change measurements
from feature-based ecological maps.
1045 Detecting Chlorophyll-a
in Lake Garda Using TOA MERIS Radiances
Claudia Giardino, Gabriele Candiani, and Eugenio Zilioli
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The performance of MERIS as a tool for mapping chlorophylla
concentrations in lake waters has been evaluated using
simulated and measured top of atmosphere radiances for
Lake Garda (Italy). MERIS observations were simulated using
hyperspectral data collected by the MIVIS imaging spectrometer
in July 2000. MIVIS data were radiometrically corrected at
the sensor altitude using the 6S radiative transfer code. The
MERIS simulation process was verified using ETM+ measurements
acquired at the same time of the MIVIS flight and
differences between simulated and actual radiance measurements
in ETM+ bands 1, 2, and 3 were about 10 percent. In
July 2003, a cloud-free MERIS image was available. MERIS
radiances of both dates were used to describe the variation
of chlorophyll-a content in the lake that was estimated
synchronously to remote observations using continuous track
fluorometer data. In 2000, when the mean value of chlorophyll-a was
about 6 mg/m3 the best performing algorithm
(RMSE = 0.58 mg/m3) was a ratio of band differences using
VIS and NIR wavelengths. In 2003, when the chlorophyll-a
concentration in the lake was very low (mean <1 mg/m3),
a single channel centered at 490 nm was the best index
in describing spatial variations of chlorophyll-a (RMSE
= 0.10 mg/m3). The results suggest that MERIS observations
are providing useful information for assessing and monitoring
chlorophyll-a distribution in lacustrine ecosystems.
1053 Lag and
Seasonality Considerations in Evaluating AVHRR NDVI Response
to Precipitation
Lei Ji and Albert J. Peters
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Assessment of the relationship between
the normalized difference vegetation index (NDVI) and precipitation
is
important in understanding vegetation and climate interaction
at a large scale. NDVI response to precipitation, however,
is difficult to quantify due to the lag and seasonality
effects, which will vary due to vegetation cover type, soils
and climate. A time series analysis was performed on
biweekly NDVI and precipitation around weather stations in
the northern and central U.S. Great Plains. Regression
models that incorporate lag and seasonality effects were
used to quantify the relationship between NDVI and lagged
precipitation in grasslands and croplands. It was found that
the time lag was shorter in the early growing season, but
longer in the mid- to late-growing season for most locations.
The regression models with seasonal adjustment indicate
that the relationship between NDVI and precipitation over the
entire growing season was strong, with R2 values of 0.69 and
0.72 for grasslands and croplands, respectively. We conclude
that vegetation greenness can be predicted using current and
antecedent precipitation, if seasonal effects are taken into
account.
1063 Triangulation
of Well-Defined Points as a Constraint for Reliable Image Matching
Qing Zhu, Jie Zhao, Hui Lin, and Jianya Gong
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This study demonstrates the utilization of the well-defined
points to improve the reliability and accuracy of image
matching. The basic principle is: (a) to triangulate a few
well-defined points within the stereo model area to form a
coarse triangulation; (b) to detect certain amount of corners
within each triangle for further matching; (c) to propagate
the matching of corner points from the reference points (i.e.,
the three triangle vertices) to obtain the best matching for
each of these corners; (d) to dynamically update the triangulation
by inserting the newly matched corner; and (e) to
further detect corners and perform matching for them until
a pre-defined criteria (the minimum size of triangle or the
largest number of points matched) is reached. Experimental
results reveal: (a) the false matching caused by the occlusion
and repetitive texture is diminished; (b) the accuracy is
improved, i.e., with a reduction of RMSE of check points
(located in different types of terrain areas) by 12 percent to
62 percent, and a reduction of the largest error by up to two
times; and (c) most building corners and boundary points of
main objects could be matched directly and accurately.
1071 Effects of Forest
Environment and Survey Protocol on GPS Accuracy
Christian Piedallu and Jean-Claude Gégout
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The aim of the study is to test GPS equipment receivers
commonly used for natural resource management, and to
quantify recording rate and positioning quality under different
conditions, the objective being to assist GPS users in their
choices.
Four factors were evaluated: (a) the type of receiver: three ranges of GPS equipment were compared; (b) forest cover effects (three covers were tested: open cover, coppice and deciduous high forest); (c) the effects of GPS survey components: the number of recordings (between 1 and 300), the Position Dilution of Precision (PDOP) thresholding (between 4 and 50), the time interval between recordings (between 1 and 15 seconds), and the differential correction effect; and (d) the season (winter and summer).
A GPS survey was carried out and a database of 140,000 readings was established, from which a large number of random rover files were extracted for each combination of factors.
It appears the only factor not to be significant is the seasonal effect. The type of equipment used and the forest cover effect both modify positioning accuracy by a factor of 2 or 3, as does the use of differential correction for Trimble receivers in open cover. Increasing the number of recordings and the time interval between recordings, and decreasing the PDOP threshold, improve precision, with a different effect according to the GPS receiver and the forest cover. The effect is generally more pronounced under high forest cover. The combined effects of GPS survey components produce significant changes in accuracy at the expense of the time spent in acquiring data.
1079 Cloud-Free Satellite
Image Mosaics with Regression Trees and Histogram Matching
E. H. Helmer and B. Ruefenacht
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Cloud-free optical satellite imagery simplifies remote sensing,
but land-cover phenology limits existing solutions to
persistent cloudiness to compositing temporally resolute,
spatially coarser imagery. Here, a new strategy for developing
cloud-free imagery at finer resolution permits simple
automatic change detection. The strategy uses regression
trees to predict pixel values underneath clouds and cloud
shadows in reference scenes from other scene dates. It then
applies improved histogram matching to adjacent scenes.
In the study area, the islands of Puerto Rico, Vieques, and
Culebra, Landsat image mosaics resulting from this strategy
permit accurate detection of land development with only
spectral data and maximum likelihood classification. Between
about 1991 and 2000, urban/built-up lands increased
by 7.2 percent in Puerto Rico and 49 percent in Vieques
and Culebra. The regression tree modeling and histogram
matching require no manual interpretation. Consequently,
they can support large volume processing to distribute
cloud-free imagery for simple change detections with common
classifiers.