Decreasing Pixel Size Research Articles

A Radiative Transfer Case Study for 3‐d cloud effects in the UV

Satellite UV mapping is usually based on the independent pixel approximation (IPA) which neglects horizontal photon transport between adjacent columns. Horizontal inhomogeneity of cloud fields therefore causes uncertainties in the derived UV radiation fields. While these effects are small for large pixel satellites, the broken‐cloud errors increase as the pixel size decreases. By comparing results of 1‐d and 3‐d UV radiative transfer calculations for three selected cloud scenes that cover a rather broad range of cloud inhomogeneity the main 3‐d cloud effects on the atmospheric UV transmission are identified and quantified in their order of magnitude. With respect to the different spatial resolutions of satellite instruments it is further shown how 3‐d cloud effects average out with increasing spatial scale. It turns out that locally the IPA cause maximum uncertainties up to ±100% for a spatial resolution of about 1 × 1 km² (e.g., AVHRR), they are reduced to ±10% for a resolution of about 15 × 15 km² and below 5% for a resolution greater than 30 km (e.g., TOMS).

Surface fractal characteristics of preferential flow patterns in field soils: evaluation and effect of image processing

In the last few decades, preferential flow has become recognized as a process of great practical significance for the transport of water and contaminants in field soils. Dyes are frequently used to visualize preferential flow pathways, and fractal geometry is increasingly applied to the characterization of these pathways via image analysis, leading to the determination of `mass' and `surface' fractal dimensions. Recent work by the authors has shown the first of these dimensions to be strongly dependent on operator choices (related to image resolution, thresholding algorithm, and fractal dimension definition), and to tend asymptotically to 2.0 for decreasing pixel size. A similar analysis is carried out in the present article in the case of the surface fractal dimension of the same stained preferential flow pathway, observed in an orchard soil. The results indicate that when the box-counting, information, and correlation dimensions of the stain pattern are evaluated via non-linear regression, they vary anywhere between 1.31 and 1.64, depending on choices made at different stages in the evaluation. Among the parameters subject to choice, image resolution does not appear to exert a significant influence on dimension estimates. A similar lack of dependency on image resolution is found in the case of a textbook surface fractal, the quadratic von Koch island. These parallel observations suggest that the observed stain pattern exhibits characteristics similar to those of a surface fractal. The high statistical significance ( R>0.99) associated with each dimension estimate lends further credence to the fractality of the stain pattern. However, when proper attention is given to the fact that the theoretical definition of the surface `fractal' dimension, in any one of its embodiments, involves the passage to a limit, the fractal character of the stain pattern appears more doubtful. Depending on the relative weight given to the available pieces of evidence, one may conclude that the stain pattern is or is not a surface fractal. However, this conundrum may or may not have practical significance. Indeed, whether or not the stain pattern is a surface fractal, the averaging method proposed in the present article to calculate surface dimensions yields relatively robust estimates, in the sense that they are independent of image resolution. These dimensions, even if they are not `fractal', may eventually play an important role in future dynamical theories of preferential flow in field soils.

An investigation of aliasing with Gaussian-diffusion modeling of SPECT system spatial resolution

The modeling of the distance-dependent spatial resolution of SPECT imaging in the projector and backprojector pair used in iterative reconstruction improves the resulting slices both quantitatively and qualitatively. By using small, spatial domain, convolution masks to model the blurring from one image plane to the next, Gaussian diffusion can markedly reduce the computational burden. The problem with Gaussian diffusion is that aliasing distorts the coefficients of the masks such that there can be a significant loss in the accuracy with which resolution is modeled. The use of integration of the probability density function of the Gaussian function over pixel boundaries as opposed to delta-function sampling, an effective value for the standard deviation of the Gaussian used in blurring from one plane to the next, and diffusion over multiple plane steps were investigated as possible methods to diminish the impact of aliasing on accuracy. For large pixel sizes the combined use of all three methods was determined to be needed to reduce the impact of aliasing on accuracy. As the pixel size decreases, the use of any of the methods results in a significant improvement in accuracy.

Iron and Manganese Distribution in Nodules from a Young Texas Vertisol

AbstractIron and Mn distributions in the black nodules from the BCk3 horizon of the microlow position of a Burleson soil, a fine, montmorillonitic, thermic, Udic Haplustert, from a terrace of the Brazos River were mapped with scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) x‐ray analysis. Spherical nodules were concentrically banded, and the irregularly shaped nodules lacked well‐developed banding of Fe and Mn. More and better expressed banding was found by decreasing pixel size used for mapping and by increasing the count time per pixel for banded nodules. Bands of higher or lower Fe and Mn concentration were not coincident. The Mn/Fe weight ratios of individual nodules analyzed by EDS were between 0.71 to 2.85. Banded nodules had a narrower range of Mn and Fe concentrations and a narrower range in Mn to Fe ratios than unbanded nodules. Barium concentration in the nodules was unusually high, averaging 30 g kg−1, and correlated strongly with Mn concentration. Some interelement correlations (e.g., Fe vs. Mn, r = −0.56; Fe vs. Al, r = −0.58) reflected partitioning into the oxide or silicate minerals of the soil. Fractured nodules revealed the interior to have only a few very small pores. The 30 nodules examined are well developed solid bodies that are expected to persist in the soil environment. They appear to contain unique and important information about the weathering history of the Burleson soil and the landscape in which it occurs. The order and expression of the banding is a relict of the redox history of the nodules, which can be used to induce the redox history of the soil.

Investigation of MR decay rates in microphantom models of trabecular bone.

MR measurements of transverse relaxation time, T2*, in trabecular bone may provide both structural and density-related information for assessment of bone mineral status in osteoporosis. Using submillimeter scale glass phantoms as simplified models of trabecular bone, we have made a quantitative investigation of the dependence of T2* decay on modeled trabecular microstructure and MR image resolution. The experimental MR data are in excellent agreement with predictions from a computer simulation. Decreasing the modeled trabecular bone volume fraction, sigma, decreases the decay rate, as expected. However, if trabecular width and spacing are both increased without changing sigma, the decay rate is unchanged. The measured decay curves closely follow the predicted dependence on trabecular orientation. The decay rates are independent of image resolution, provided that the pixel dimensions are larger than the intertrabecular spacing. For smaller pixel sizes, the decay rate decreases with decreasing pixel size.

Practical limits for optical test of spatial resolution in advanced imaging devices

Decreasing pixel size, perfection of isolation, and improved dynamic range of advanced imaging devices requires an adequate optical arrangement to check crosstalk behavior within the pixel matrix. The Fraunhofer diffraction pattern; an inherent feature of any focused circular beam, leads to unintended direct illumination of the neighbors even if accurately centered. A Simple setup has been applied to determine the beam profile focused down to the subwavelength range. We propose to characterize the beam focusing for spatial resolution tests by the first zero to-zero width independent of peak intensity. We provide an estimate of minimum requirements to be fulfilled in order to test for a given crosstalk and demonstrate their importance on filled trench-isolated pixels.

Preliminary evidence for the influence of physiography and scale upon the autocorrelation function of remotely sensed data

Previously established results demonstrate that LANDSAT data are autocorrelated and can be described by a univariate linear stochastic process known as auto-regressive-integrated-moving-average model of degree 1, 0, 1 or ARIMA (1, 0, 1). This model has two coefficients of interest for interpretation phi(1) and theta(1). In a comparison of LANDSAT thematic mapper simulator (TMS) data and LANDSAT MSS data several results were established: (1) The form of the relatedness as described by this model is not dependent upon system look angle or pixel size. (2) The phi(1) coefficient increases with decreasing pixel size and increasing topographic complexity. (3) Changes in topography have a greater influence upon phi(1) than changes in land cover class. (4) The theta(1) seems to vary with the amount of atmospheric haze. These patterns of variation in phi(1) and theta(1) are potentially exploitable by the remote sensing community to yield stochastically independent sets of observations, characterize topography, and reduce the number of bytes needed to store remotely sensed data.