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1.
A wide-angle airborne laser ranging system (WA-ALRS) is developed at the Institut Géographique National (IGN), France, with
the aim of providing a new geodesy technique devoted to large (100 km2) networks with a high density (1 km−2) of benchmarks. The main objective is to achieve a 1-mm accuracy in relative vertical coordinates from aircraft measurements
lasting a few hours. This paper reviews the methodology and analyzes the first experimental data achieved from a specific
ground-based experiment. The accuracy in relative coordinate estimates is studied with the help of numerical simulations.
It is shown that strong accuracy limitations arise with a small laser beam divergence combined with short range measurements
when relatively few simultaneous range data are produced. The accuracy is of a few cm in transverse coordinates and a few
mm in radial coordinates. The results from ground-based experimental data are fairly compatible with these predictions. The
use of a model for systematic errors in the vehicle trajectory is shown to be necessary to achieve such a high accuracy. This
work yields the first complete validation of modelization and methodology of this technique. An accuracy better than 1 mm
and a few mm in vertical and horizontal coordinates, respectively, is predicted for aircraft experiments.
Received: 19 June 1997 / Accepted: 17 February 1998 相似文献
2.
R. P. Singh S. Rovshan S. K. Goroshi S. Panigrahy J. S. Parihar 《Journal of the Indian Society of Remote Sensing》2011,39(3):345-353
The monitoring of terrestrial carbon dynamics is important in studies related with global climate change. This paper presents
results of the inter-annual variability of Net Primary Productivity (NPP) from 1981 to 2000 derived using observations from
NOAA-AVHRR data using Global Production Efficiency Model (GloPEM). The GloPEM model is based on physiological principles and
uses the production efficiency concept, in which the canopy absorption of photosynthetically active radiation (APAR) is used
with a conversion “efficiency” to estimate Gross Primary Production (GPP). NPP derived from GloPEM model over India showed
maximum NPP about 3,000 gCm−2year−1 in west Bengal and lowest up to 500 gCm−2year−1 in Rajasthan. The India averaged NPP varied from 1,084.7 gCm−2year−1 to 1,390.8 gCm−2year−1 in the corresponding years of 1983 and 1998 respectively. The regression analysis of the 20 year NPP variability showed significant
increase in NPP over India (r = 0.7, F = 17.53, p < 0.001). The mean rate of increase was observed as 10.43 gCm−2year−1. Carbon fixation ability of terrestrial ecosystem of India is increasing with rate of 34.3 TgC annually (t = 4.18, p < 0.001). The estimated net carbon fixation over Indian landmass ranged from 3.56 PgC (in 1983) to 4.57 PgC (in 1998). Grid
level temporal correlation analysis showed that agricultural regions are the source of increase in terrestrial NPP of India.
Parts of forest regions (Himalayan in Nepal, north east India) are relatively less influenced over the study period and showed
lower or negative correlation (trend). Finding of the study would provide valuable input in understanding the global change
associated with vegetation activities as a sink for atmospheric carbon dioxide. 相似文献
3.
Robert Odolinski 《GPS Solutions》2012,16(2):147-155
Temporal correlation in network real-time kinematic (RTK) data exists due to unmodeled multipath and atmospheric errors, in
combination with slowly changing satellite constellation. If this correlation is neglected, the estimated uncertainty of the
coordinates might be too optimistic. In this study, we compute temporal correlation lengths for network RTK positioning, i.e.,
the appropriate time separation between the measurements. This leads to more realistic coordinate uncertainty estimates, and
an appropriate surveying strategy to control the measurements can be designed. Two methods to estimate temporal correlation
lengths are suggested. Several monitor stations that utilize correction data from two SWEPOSTM Network RTK services, a standard service and a project-adapted service with the mean distance between the reference stations
of approximately 70 and 10–20 km, are evaluated. The correlation lengths for the standard service are estimated as 17 min
for the horizontal component and 36–37 min for the vertical component. The corresponding estimates for the project-adapted
service are 13–17 and 13–16 min, respectively. According to the F test, the proposed composite first-order Gauss–Markov autocovariance function shows a significantly better least-squared
fit to data compared to the commonly used one-component first-order Gauss–Markov model. A second suggested method is proposed
that has the potential of providing robust correlation lengths without the need to fit a model to the computed autocovariance
function. 相似文献
4.
Stochastic assessment of GPS carrier phase measurements for precise static relative positioning 总被引:17,自引:11,他引:17
Global positioning system (GPS) carrier phase measurements are used in all precise static relative positioning applications.
The GPS carrier phase measurements are generally processed using the least-squares method, for which both functional and stochastic
models need to be carefully defined. Whilst the functional model for precise GPS positioning is well documented in the literature,
realistic stochastic modelling for the GPS carrier phase measurements is still both a controversial topic and a difficult
task to accomplish in practice. The common practice of assuming that the raw GPS measurements are statistically independent
in space and time, and have the same accuracy, is certainly not realistic. Any mis-specification in the stochastic model will
inevitably lead to unreliable positioning results. A stochastic assessment procedure has been developed to take into account
the heteroscedastic, space- and time-correlated error structure of the GPS measurements. Test results indicate that the reliability
of the estimated positioning results is improved by applying the developed stochastic assessment procedure. In addition, the
quality of ambiguity resolution can be more realistically evaluated.
Received: 13 February 2001 / Accepted: 3 September 2001 相似文献
5.
Change of river channel and bank erosion of the burhi dihing river (assam), assessed using remote sensing data and gis 总被引:1,自引:0,他引:1
The Burhi Dining river flows in a meandering course for about 220 km through alluvial plains of Assam including a short rocky
and hilly tract in between. Sequential changes in the position of banklines of the river due to consistent bank erosion have
been studied from Survey of India topographic maps of 1934 and 1972, and digital satellite data of 2001 and 2004 using GIS.
Two broad kinds of changes have been observed, e.g. alteration of direction of flow due to neck cut-off and progressive gradual
change of the meander bends that accounts for translational, lateral, rotational, extensional and other types of movement
of the meander bends. Study of bankline shift due to the bank erosion has been carried out for the periods 1934–1972, 1972–2001,
2001–2004 and 1934–2004 at 13 segments spaced at 5′ longitude interval (average 15 km) as the river course trends nearly east
to west. The amounts of the bank area lost due to erosion and gained due to sediment deposition are estimated separately.
The total area eroded in both banks during 1934–1972 was more (26.796 km2) as compared to sediment deposition (19.273 km2), whereas total sediment deposition was more (34.61 km2) during 1972-2001 as compared to erosion (23.152 km2). Erosion was again more in 2001–2004 (7.568 km2) as compared to sediment deposition (2.493 km2). During the entire period (1934–2004) of study the overall erosion on the both banks was 31.169 km2 and overall sediment deposition was 30.101 km2. The highest annual rates of bank erosion as well as bank building of the river are 21055.47 m2/km in 2001–2004 and 9665.81 m2/km in 1972-2001, respectively. Similarly the highest average annual rates of erosion as well as sediment deposition in both
banks are observed during 2001–2004 and 1972–2001, respectively. The hard rocks of the hilly tract situated in between result
in development of entrenched meandering and this tract has suffered minimum bank erosion. 相似文献
6.
Fast transform from geocentric to geodetic coordinates 总被引:3,自引:0,他引:3
T. Fukushima 《Journal of Geodesy》1999,73(11):603-610
A new iterative procedure to transform geocentric rectangular coordinates to geodetic coordinates is derived. The procedure
solves a modification of Borkowski's quartic equation by the Newton method from a set of stable starters. The new method runs
a little faster than the single application of Bowring's formula, which has been known as the most efficient procedure. The
new method is sufficiently precise because the resulting relative error is less than 10−15, and this method is stable in the sense that the iteration converges for all coordinates including the near-geocenter region
where Bowring's iterative method diverges and the near-polar axis region where Borkowski's non-iterative method suffers a
loss of precision.
Received: 13 November 1998 / Accepted: 27 August 1999 相似文献
7.
A wide-angle airborne laser ranging system has been developed for the determination of relative heights of ground-based benchmarks
in regional-scale networks (typically 100 laser reflectors spread over 100 km2). A first prototype demonstrated a 1–2 mm accuracy in radial distance measurement in a ground-based experiment in 1995. The
first aircraft experiment was conducted in 1998, over a small area (1 km2) equipped with a network of 64 benchmarks. The instrument was modified before that experiment, in order to minimize echo
superimposition due to the high density of benchmarks. New data processing algorithms have been developed, for the deconvolution
of strongly overlapped echoes and a high a priori uncertainty in the aircraft flight path, and for the estimation of benchmark
coordinates. A special methodology has been developed for the parameterization of these algorithms and of outlier detection
tests. From a total of 2×104 pseudo-range measurements, that have been acquired from two flights composed of 30 legs each, only 3×103 remain after outlier detection. A positioning accuracy of 1.5 cm in the vertical coordinate (2.1 cm in the difference between
the two flights) has been achieved. It is shown that the errors are normally distributed, with a nearly zero mean, and are
consistent with the a posteriori uncertainty. It is also shown that the accuracy is limited mainly by the sensitivity of the
photodetector used for this experiment (due to reduced response time). Another limiting factor is the effect of aircraft attitude
changes during the measurements, which produces additional uncertainties in absolute distance measurements. It is planned
to test new photodetectors with high internal gains. These should provide, in future experiments with smaller benchmark density,
an improvement in signal-to-noise ratio of a factor of 5–10, leading to sub-centimeter vertical positioning accuracy.
Received: 19 June 2001 / Accepted: 3 January 2002 相似文献
8.
大光斑激光雷达数据已广泛应用于森林冠层高度提取,但通常仅限于地形坡度小于20°的平缓地区。在地形坡度大于20°的陡峭山区,地形引起的波形展宽使得地面回波和植被回波信息混合在一起,给森林冠层高度提取带来巨大挑战。本文利用激光雷达回波模型和地形信息,提出了一种模型辅助的坡地森林冠层高度反演算法。该方法以激光雷达回波信号截止点为参考,定义了波形高度指数H50和H75,使用激光雷达回波模型与已知地形信息模拟裸地的激光雷达回波,将裸地回波信号截止点与森林激光雷达回波信号截止点对齐,利用裸地回波计算常用的波形相对高度指数RH50和RH75,对森林冠层高度进行反演。并与高斯波形分解法和波形参数法的反演结果进行了比较。研究结果表明:(1)利用所提取的波形指数RH50和RH75对胸高断面积加权平均高(Lorey’s height)进行了估算,在坡度小于20°时,高斯波形分解法、波形参数法和模型辅助法的估算结果与实测值线性拟合的相关系数(R2)分别为0.70,0.78和0.98,对应的均方根误差(RMSE)分别为2.90 m,2.48 m和0.60 m,模型辅助法略优于其他两种方法;(2)在坡度大于20°时,高斯波形分解法、波形参数法和模型辅助法的R2分别为0.14,0.28和0.97,相应的RMSE分别为4.93 m,4.53 m和0.81 m,模型辅助法明显优于其他两种方法;(3)在0°—40°时,模型辅助法对Lorey’s height估算结果与实测值的R2为0.97,RMSE为0.80 m。本研究提出的模型辅助法具有更好的地形适应性,在0°—40°的坡度范围内具备对坡地森林冠层高度反演的潜力。 相似文献
9.
Accuracy of GPS-derived relative positions as a function of interstation distance and observing-session duration 总被引:6,自引:0,他引:6
Ten days of GPS data from 1998 were processed to determine how the accuracy of a derived three-dimensional relative position
vector between GPS antennas depends on the chord distance (denoted L) between these antennas and on the duration of the GPS observing session (denoted T). It was found that the dependence of accuracy on L is negligibly small when (a) using the `final' GPS satellite orbits disseminated by the International GPS Service, (b) fixing
integer ambiguities, (c) estimating appropriate neutral-atmosphere-delay parameters, (d) 26 km ≤ L ≤ 300 km, and (e) 4 h ≤T ≤ 24 h. Under these same conditions, the standard error for the relative position in the north–south dimension (denoted S
n
and expressed in mm) is adequately approximated by the equation S
n
=k
n
/T
0.5 with k
n
=9.5 ± 2.1 mm · h0.5 and T expressed in hours. Similarly, the standard errors for the relative position in the east–west and in the up-down dimensions
are adequately approximated by the equations S
e
=k
e
/T
0.5 and S
u
=k
u
/T
0.5, respectively, with k
e
=9.9 ± 3.1 mm · h0.5 and k
u
=36.5 ± 9.1 mm · h0.5.
Received: 5 February 2001 / Accepted: 14 May 2001 相似文献
10.
Assessment of groundwater potential zones using GIS technique 总被引:1,自引:0,他引:1
A case study was conducted to find out the groundwater potential zones in Kattakulathur block, Tamil Nadu, India with an aerial
extent of 360.60 km2. The thematic maps such as geology, geomorphology, soil hydrological group, land use / land cover and drainage map were prepared
for the study area. The Digital Elevation Model (DEM) has been generated from the 10 m interval contour lines (which is derived
from SOI, Toposheet 1:25000 scale) and obtained the slope (%) of the study area. The groundwater potential zones were obtained
by overlaying all the thematic maps in terms of weighted overlay methods using the spatial analysis tool in ArcGIS 9.2. During
weighted overlay analysis, the ranking has been given for each individual parameter of each thematic map and weights were
assigned according to the influence such as soil −25%, geomorphology − 25%, land use/land cover −25%, slope − 15%, lineament
− 5% and drainage / streams − 5% and find out the potential zones in terms of good, moderate and poor zones with the area
of 49.70 km2, 261.61 km2 and 46.04 km2 respectively. The potential zone wise study area was overlaid with village boundary map and the village wise groundwater
potential zones with three categories such as good, moderate and poor zones were obtained. This GIS based output result was
validated by conducting field survey by randomly selecting wells in different villages using GPS instruments. The coordinates
of each well location were obtained by GPS and plotted in the GIS platform and it was clearly shown that the well coordinates
were exactly seated with the classified zones. 相似文献
11.
Mohammad Asadullah Khan 《Journal of Geodesy》1973,47(3):227-235
An intrresting variation on the familiar method of determining the earth's equatorial radius ae, from a knowledge of the earth's equatorial gravity is suggested. The value of equatorial radius thus found is 6378,142±5
meters. The associated parameters are GM=3.986005±.000004 × 1020 cm3 sec-−2 which excludes the relative mass of atmosphere ≅10−6 ξ GM, the equatorial gravity γe 978,030.9 milligals (constrained in this solution by the Potsdam Correction of 13.67 milligals as the Potsdam Correction
is more directly, orless indirectly, measurable than the equatorial gravity) and an ellipsoidal flattening of f=1/298.255. 相似文献
12.
A 2×2 arc-minute resolution geoid model, CARIB97, has been computed covering the Caribbean Sea. The geoid undulations refer
to the GRS-80 ellipsoid, centered at the ITRF94 (1996.0) origin. The geoid level is defined by adopting the gravity potential
on the geoid as W
0=62 636 856.88 m2/s2 and a gravity-mass constant of GM=3.986 004 418×1014 m3/s2. The geoid model was computed by applying high-frequency corrections to the Earth Gravity Model 1996 global geopotential
model in a remove-compute-restore procedure. The permanent tide system of CARIB97 is non-tidal. Comparison of CARIB97 geoid
heights to 31 GPS/tidal (ITRF94/local) benchmarks shows an average offset (h–H–N) of 51 cm, with an Root Mean Square (RMS) of 62 cm about the average. This represents an improvement over the use of a global
geoid model for the region. However, because the measured orthometric heights (H) refer to many differing tidal datums, these comparisons are biased by localized permanent ocean dynamic topography (PODT).
Therefore, we interpret the 51 cm as partially an estimate of the average PODT in the vicinity of the 31 island benchmarks.
On an island-by-island basis, CARIB97 now offers the ability to analyze local datum problems which were previously unrecognized
due to a lack of high-resolution geoid information in the area.
Received: 2 January 1998 / Accepted: 18 August 1998 相似文献
13.
Milan Burša Steve Kenyon Jan Kouba Zdislav Šíma Viliam Vatrt Vojtěch Vítek Marie Vojtíšková 《Journal of Geodesy》2007,81(2):103-110
The TOPEX/Poseidon (T/P) satellite alti- meter mission marked a new era in determining the geopotential constant W
0. On the basis of T/P data during 1993–2003 (cycles 11–414), long-term variations in W
0 have been investigated. The rounded value W
0 = 62636856.0 ± 0.5) m
2
s
−2 has already been adopted by the International Astronomical Union for the definition of the constant L
G
= W
0/c
2 = 6.969290134 × 10−10 (where c is the speed of light), which is required for the realization of the relativistic atomic time scale. The constant L
G
, based on the above value of W
0, is also included in the 2003 International Earth Rotation and Reference Frames Service conventions. It has also been suggested
that W
0 is used to specify a global vertical reference system (GVRS). W
0 ensures the consistency with the International Terrestrial Reference System, i.e. after adopting W
0, along with the geocentric gravitational constant (GM), the Earth’s rotational velocity (ω) and the second zonal geopotential
coefficient (J
2) as primary constants (parameters), then the ellipsoidal parameters (a,α) can be computed and adopted as derived parameters. The scale of the International Terrestrial Reference Frame 2000 (ITRF2000)
has also been specified with the use of W
0 to be consistent with the geocentric coordinate time. As an example of using W
0 for a GVRS realization, the geopotential difference between the adopted W
0 and the geopotential at the Rimouski tide-gauge point, specifying the North American Vertical Datum 1988 (NAVD88), has been
estimated. 相似文献
14.
Oliver Montenbruck Miquel Garcia-Fernandez Yoke Yoon Steffen Schön Adrian Jäggi 《GPS Solutions》2009,13(1):23-34
Phase center variations of the receiver and transmitter antenna constitute a remaining uncertainty in the high precision orbit
determination (POD) of low Earth orbit (LEO) satellites using GPS measurements. Triggered by the adoption of absolute phase
patterns in the IGS processing standards, a calibration of the Sensor Systems S67-1575-14 antenna with GFZ choke ring has
been conducted that serves as POD antenna on various geodetic satellites such as CHAMP, GRACE and TerraSAR-X. Nominal phase
patterns have been obtained with a robotic measurement system in a field campaign and the results were used to assess the
impact of receiver antenna phase patterns on the achievable positioning accuracy. Along with this, phase center distortions
in the actual spacecraft environment were characterized based on POD carrier phase residuals for the GRACE and TerraSAR-X
missions. It is shown that the combined ground and in-flight calibration can improve the carrier phase modeling accuracy to
a level of 4 mm which is close to the pure receiver noise. A 3.5 cm (3D rms) consistency of kinematic and reduced dynamic
orbit determination solutions is achieved for TerraSAR-X, which presumably reflects the limitations of presently available
GPS ephemeris products. The reduced dynamic solutions themselves match the observations of high grade satellite laser ranging
stations to 1.5 cm but are potentially affected by cross-track biases at the cm-level. With respect to the GPS based relative
navigation of TerraSAR-X/TanDEM-X formation, the in-flight calibration of the antenna phase patterns is considered essential
for an accurate modeling of differential carrier phase measurements and a mm level baseline reconstruction.
相似文献
Oliver MontenbruckEmail: |
15.
Improving the GNSS positioning stochastic model in the presence of ionospheric scintillation 总被引:1,自引:0,他引:1
M. Aquino J. F. G. Monico A. H. Dodson H. Marques G. De Franceschi L. Alfonsi V. Romano M. Andreotti 《Journal of Geodesy》2009,83(10):953-966
Ionospheric scintillations are caused by time- varying electron density irregularities in the ionosphere, occurring more often
at equatorial and high latitudes. This paper focuses exclusively on experiments undertaken in Europe, at geographic latitudes
between ~50°N and ~80°N, where a network of GPS receivers capable of monitoring Total Electron Content and ionospheric scintillation parameters was
deployed. The widely used ionospheric scintillation indices S4 and sj{\sigma_{\varphi}} represent a practical measure of the intensity of amplitude and phase scintillation affecting GNSS receivers. However, they
do not provide sufficient information regarding the actual tracking errors that degrade GNSS receiver performance. Suitable
receiver tracking models, sensitive to ionospheric scintillation, allow the computation of the variance of the output error
of the receiver PLL (Phase Locked Loop) and DLL (Delay Locked Loop), which expresses the quality of the range measurements
used by the receiver to calculate user position. The ability of such models of incorporating phase and amplitude scintillation
effects into the variance of these tracking errors underpins our proposed method of applying relative weights to measurements
from different satellites. That gives the least squares stochastic model used for position computation a more realistic representation,
vis-a-vis the otherwise ‘equal weights’ model. For pseudorange processing, relative weights were com- puted, so that a ‘scintillation-mitigated’
solution could be performed and compared to the (non-mitigated) ‘equal weights’ solution. An improvement between 17 and 38%
in height accuracy was achieved when an epoch by epoch differential solution was computed over baselines ranging from 1 to
750 km. The method was then compared with alternative approaches that can be used to improve the least squares stochastic
model such as weighting according to satellite elevation angle and by the inverse of the square of the standard deviation
of the code/carrier divergence (sigma CCDiv). The influence of multipath effects on the proposed mitigation approach is also
discussed. With the use of high rate scintillation data in addition to the scintillation indices a carrier phase based mitigated
solution was also implemented and compared with the conventional solution. During a period of occurrence of high phase scintillation
it was observed that problems related to ambiguity resolution can be reduced by the use of the proposed mitigated solution. 相似文献
16.
Varghese Ouseph Alappat Ashok Kumar Joshi Y. V. N. Krishnamurthy 《Journal of the Indian Society of Remote Sensing》2011,39(4):583-589
Optical remote sensing data have been extensively used to derive biophysical properties that relate forest type and composition.
However, stand density, stand height and stand volume cannot be estimated directly from optical remote sensing data owing
to poor sensitivity between these parameters and spectral reflectance. The ability of microwave energy to penetrate within
forest vegetation makes it possible to extract information on both the crown and trunk components from radar data. The type
of polarization employed determines the radar response to the various shapes and orientations of the scattering mechanisms
within the canopy or trunk. This study mainly presents experimental results obtained with airborne E-SAR using polarimetric
C and L bands over the tropical dry deciduous forest of Chandrapur Forest Division, Maharashtra. A detailed documentation
of the relationship between SAR C & L bands backscattering and forest stand variables has been provided in the present study
through linear correlation. Linear correlation of the single channel SAR derived estimates with the field measured means show
a good correlation between L HV backscattering coefficient with stand volume (r2 = 0.71) and L HH backscattering coefficient with stand density (r2 = 0.75). The results imply that SAR data has significant potential for stand menstruation in operational forestry. 相似文献
17.
The 2 arc-minute × 2 arc-minute geoid model (GEOID96) for the United States supports the conversion between North American
Datum 1983 (NAD 83) ellipsoid heights and North American Vertical Datum 1988 (NAVD 88) Helmert heights. GEOID96 includes information
from global positioning system (GPS) height measurements at optically leveled benchmarks. A separate geocentric gravimetric
geoid, G96SSS, was first calculated, then datum transformations and least-squares collocation were used to convert from G96SSS
to GEOID96.
Fits of 2951 GPS/level (ITRF94/NAVD 88) benchmarks to G96SSS show a 15.1-cm root mean square (RMS) around a tilted plane (0.06 ppm,
178∘ azimuth), with a mean value of −31.4 cm (15.6-cm RMS without plane). This mean represents a bias in NAVD 88 from global mean
sea level, remaining nearly constant when computed from subsets of benchmarks. Fits of 2951 GPS/level (NAD 83/NAVD 88) benchmarks
to GEOID96 show a 5.5-cm RMS (no tilts, zero average), due primarily to GPS error. The correlated error was 2.5 cm, decorrelating
at 40 km, and is due to gravity, geoid and GPS errors. Differences between GEOID96 and GEOID93 range from −122 to +374 cm
due primarily to the non-geocentricity of NAD 83.
Received: 28 July 1997 / Accepted: 2 September 1998 相似文献
18.
Bradford W. Parkinson 《Journal of Geodesy》1979,53(2):89-108
In October 1978, the third of a series of prototype navigational satellites of an entirely new design was successfully launched.
By the end of the first phase (1979) there will be six of these “NAVSTAR” satellites. They constitute one piece of a major
development thrust to deploy a revolutionary navigation system called the Global Positioning System (GPS), or NAVSTAR. This
system, being developed by the Air Force for the joint use of al components of the Department of Defense, has been under development
since 1972. The purpose of this article is to describe the system, indicate the requirements which have caused the Department
of Defense to spend many millions of dollars on the development, outline how the geodetic community can potentially use it,
present the test results that have been accumulated to date and finally, indicate what future plans lie in store for this
new system. 相似文献
19.
An inverse Poisson integral technique has been used to determine a gravity field on the geoid which, when continued by analytic
free space methods to the topographic surface, agrees with the observed field. The computation is performed in three stages,
each stage refining the previous solution using data at progressively increasing resolution (1o×1o, 5′×5′, 5/8′×5/8′) from a decreasing area of integration. Reduction corrections are computed at 5/8′×5/8′ granularity by
differencing the geoidal and surface values, smoothed by low-pass filtering and sub-sampled at 5′ intervals. This paper discusses
1o×1o averages of the reduction corrections thus obtained for 172 1o×1o squares in western North America.
The 1o×1o mean reduction corrections are predominantly positive, varying from −3 to +15mgal, with values in excess of 5mgal for 26 squares. Their mean andrms values are +2.4 and 3.6mgal respectively and they correlate well with the mean terrain corrections as predicted byPellinen in 1962. The mean andrms contributions from the three stages of computation are: 1o×1o stage +0.15 and 0.7mgal; 5′×5′ stage +1.0 and 1.6mgal; and 5/8′×5/8′ stage +1.3 and 1.8mgal. These results reflect a tendency for the contributions to become larger and more systematically positive as the wavelengths
involved become shorter. The results are discussed in terms of two mechanisms; the first is a tendency for the absolute values
of both positive and negative anomalies to become larger when continued downwards and, the second, a non-linear rectification,
due to the correlation between gravity anomaly and topographic height, which results in the values continued to a level surface
being systematically more positive than those on the topography. 相似文献
20.
R. B. Gajjar A. M. Shekh A. J. Dave C. T. Patel R. S. Parmar N. K. Patel J. G. Talati 《Journal of the Indian Society of Remote Sensing》2005,33(1):147-153
A field experiment was conducted on wheat crop during rabi seasons of 1995–96, 1996–97 and 1997–98 to study the spectral response
of wheat crop (between 490 to 1080 nm) under water and nutrient stress condition. An indigenously developed ground truth radiometer
having narrow band in visible and near infrared region (490 – 1080 nm) was used. Vegetation indices derived using different
band combinations and related to crop growth parameters. The near infrared spectral region of 710 – 1025 nm was found most
important for monitoring stress condition. Relationship has been developed between crop growth parameters and vegetation indices.
Leaf Area Index (LAI) and chlorophyll could be predicted by knowing different reflectance ratios at milking stage of crop
with R2 value of 0.78 and 0.89, respectively. Dry biomass (DBM), Plant Water Content (PWC) and grain yield are also significantly
related with reflectance ratios at flowering stage of crop with R2 value of 0.90, 0.98 and 0.74, respectively. 相似文献