杂地区。方格网法、断面法、DEM法是土木工程中三种常用的土方测量计算方法，具体选择哪种方法要根据具体的地形特点、精度要求等确定，不论哪种方法，采样点的密度是决定土方量精度的关键。
（四）等高线法
等高线法计算土方量适合于地形坡度均匀的地方，其等高线必须闭合，否则难以计算。因此，等高线法适合于地面坡度大，地物少的地方。此法操作比较简便，确定设计高程之后，计算填挖方的平均深度，计算填方挖方的面积，根据前面的结果可以计算出相应的土方量。
4.2结论
由于地形的复杂性，在外业测量数据相同的情况下，计算结果的精度取决于内业建模方法。DEM法和等高线法采用三角网进行建模，提取的地面高程精度高。等高线法虽然也是三角网建模，但是必须是坡度均与的地方，其等高线必须闭合，否则难以计算。DEM法则可在任意场地上计算，精度容易保证。DEM法能够计算出设计面是水平面、倾斜面和不规则面等的土方量，因此，该方法在确定任意俩个不规则地面之间的土方量及控制施工进度方面的作用非常大。
断面法和方格法都是类似人工模拟地面的方法，两个高程碎步点之间的坡度被看作均匀坡度，其高程的提取不准确，特别是方格边长较长时，精度较低。相对来说，断面法由于提取断面坡度线比方格法确定高程的精度高，所以其计算土方量的精度也比方格法高。
从理论上分析可知，在小面积土方测量（距离小于100m）中，水准仪测量地面高程和土方量的精度都比光学经纬仪高，但由于定线、量距地误差较大，计算土方量的精度就不如全站仪，因而适合于平坦的地形测量。用经纬仪代替水准仪测量，可以提高测量的速度，适合于精度要求不高的丘陵地区的土方测量。全站仪在外业测量地面高程的精度虽不如水准仪，但优于经纬仪。全站仪测量的土方量精度比水准仪和经纬仪都高，分别是水准仪的2倍和经纬仪都高，分别是水准仪的2倍和经纬仪的4倍。随着高精度全站仪的普及，土方测量精度也相应提高。
结果表明，用全站仪采集数据、DEM法计算土方量，既能提高精度又可以提高速度，适合于任何场地使用，是目前土方量测量精度最高、测量速度最快的一种方法。但方格法、等高线法、断面法和DEM法计算土方量的方法各有特色，应跟据具体情况选择运用。
通过对以上几种土方量计算方法的介绍比较，我们可以看到一下几点： (1)在较为平坦的平原区和地形起伏不大的场地，宜采用方格网法。这种方法计算的数据量小，计算速度快，省却了DEM法庞大的数据存储量；
(2)在狭长地带，比如公路、水渠等则适宜使用断面法进行计算土方量； (3)在地形起伏较大、精度要求高的一些山区则需要用到DEM的计算方法。
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但是也要考虑到，如果地图本身数据量大，数据储存量的问题；
(4)等高线法因其操作简单通常被施工单位采用，但是误差较大。当地面起伏较大、坡度变化较多时，可采用此法估算土方量，尤其在地形图精度要求较高时更为适用。
总之，在对土方量进行计算时，要考虑到地形特征、精度要求以及施工成本等方面的情况，选择合适的计算方法，达到最优的目的。另外运用CASS7.1软件有许多值得注意的地方，否则容易引起较大误差，下面就列出一些注意事项：
(1)进行土方计算之前，大多都需要首先用复合线画出计算范围，一定要闭合，但尽量不要拟合。因为拟合过的曲线在进行土方计算时会用折线迭代，影响计算结果的精度。
(2)在利用DTM法进行土方计算时，应该对已经生成的三角网进行必要的添加和删除，使结果更接近实际地貌。
(3)用读取图上三角网的方法进行土方量计算时，不要求给定区域边界，系统会分析所有被选取三角形，因此在选择三角形时一定要注意不要漏选或多选，否则计算结果有误，且很难检查出问题所在。
(4)用格网法计算土方量，设计面可以是水平的，也可以是倾斜的。对于平场，可根据提示直接输入设计高程，对于单一倾斜场地，应首先在坡地线上点取高程相等的两点联成基准线，以此为基础，指定斜坡变化方向，并输入倾斜坡度。根据软件提示进行土方计算。
(5)在“方格宽度”栏，输入方格网的宽度，这是每个方格的边长，默认值为20 m。由原理可知，方格的宽度越小，计算精度越高。但如果给的值太小，超过了野外采集的点的密度也是没有实际意义的。同理，在区域土方平衡中边界上的取样密度，也是如此。
(6)道路断面法和场地断面法土方计算不同之处是，前者需要在断面参数对话框中输入与道路相关的各种参数，而对于后者，此对话框中，只有坡度等参数才可选。
(7)如果道路设计时该区段的中桩高程全部一样，就不需要下一步的编辑工作了。但实际上，有些断面的设计高程可能和其他的不一样，这样就需要手工编辑这些断面。
(8)利用断面法计算二期间土方量，用一个里程文件生成的纵横断面图，另外一期的横断面线需要使用“工程应用”菜单下的“断面法土方计算”子菜单中的“图上添加断面线”命令来实现。
(9)计算任意两条等高线之间的土方量，所选等高线必须闭合。由于两条等高线所围面积可求，两条等高线之间的高差已知，可求出这两条等高线之间的土方量。
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参考文献
[1]周国树.现代测绘技术及应用.中国水利水电出版社，2009 [2]张正禄等编著.工程测量.武汉：武汉大学出版社，2005
[3]柳长顺、齐实、杜丽娟等.土地整理项目土方量计算方法.中国水土保持科学，2003 [4]李志林、朱庆.数字高程模型.武汉：武汉大学出版社，2000
[5]周越轩、刘学军、杨治洪.基于DTM的土方工程计算与精度分析.长沙交通学院学报.2000 [6]张婷婷.王铁良.土方量计算方法研究.安徽农业科学.2006 [7]邓子胜.对场地平整土方量计算公式的分析与建议.施工技术.2004 [8]王斌.DEM土方量计算的误差探讨.铁路航测.1999 [9]程传林.土方量计算方法比较.铜陵学院学报.2004
[10]方俊.大型场地平整土方量计算及其设计标高估算.重庆建筑大学学报.2004 [11]邓子胜.对场地平整土方量计算公式的分析建议.施工技术.2004 [12]林观土.不同土方测量方法的比较与评价.广东农业科学.2007 [13]林观土.CASS7.0软件在土方量计算中的应用.农业网络信息.2007 [14]徐宇飞.数字测图技术.黄河水利出版社.2006
[15]罗德仁、邹自力、汤江龙.工程土方量计算比较分析.东华理工学院学报.2005 [16]刘建英.南方CASS软件土方量计算方法的探讨以及特殊地貌土方量的计算.城市勘
测.2008
[17]M.D.Glezin. Use of integral curves of earthwork volumes. Kluwer Academic Publishers.1967
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外文资料
Mining GPS measurement errors
Abstract: Currently,GPS applications in the measurement more widely, and has good accuracy, to complete most of the measurements. Paper, according to the principles of GPS measurements, with an engineering example, the GPS measurement error
sources in a detailed analysis, and discusses measures to reduce the GPS positioning error.
Key words: GPS; measurement error; sources; measures
Passive GPS is a navigation system, users receive the satellite signals to obtain satellite ephemeris, the distance between the user and satellite clock correction
parameters and other data, measured by random software to calculate the coordinates of points to determine the user's exact location. But in the process of positioning the satellite signal will be a variety of adverse external factors and their own, thus
positioning resulted in errors. These factors in addition to the location of the satellite itself and allowed the clock to carry, but also including measurement error factors. 1 GPS error sources
GPS measurement error can be divided into three categories according to their sources: the error of GPS signal itself, including the orbit errors, SA and AS effects; GPS signal transmission errors, including the ionospheric delay, tropospheric delay, multipath propagation, and jump the whole week ; GPS receiver errors, including clock errors, the deviation between channels, phase-locked loop delay, error code tracking loop, Antenna phase center offsets and so on. 1.1 Ionospheric Refraction
When the GPS signal through the ionosphere, the satellite signal will change the path, velocity will also change. Therefore, the signal propagation time through the vacuum speed of light and the distance will be calculated from the satellite to the receiver with the actual geometric distance are different, this difference is called the ionospheric refraction error. The size of the ionospheric correction is mainly affected by the number of total electronic and signal frequency. Carrier phase measurements of ionospheric refraction correction and the pseudo-range corrections when measuring the same size, opposite sign, for the GPS signal, the distance correction in the zenith direction up to 50 m, the direction towards the horizon (elevation angle is 20 °) up to 150 m, the results must be corrected for positioning, otherwise it will seriously degrade the accuracy of observations. 1.2Tropospheric refraction error
Troposphere is 40 km altitude to the ground from the atmosphere.
Electromagnetic signal through the atmosphere when tropospheric propagation velocity will change, leading to the results of GPS ranging error and the actual
distance, known as tropospheric refraction errors. Tropospheric refraction error of the size of the main and the temperature, humidity, air pressure, the satellite elevation angle and other factors. The impact of tropospheric refraction generally distributed in
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2 ~ 30 m. For short range (less than 20 km) GPS baseline measurements, GPS receivers can be effectively reduced the differential between the tropospheric refraction errors. For long distance, through observation of the weather elements
(such as temperature, humidity, pressure), according to the correction model has been corrected, it can also be introduced in the model differential tropospheric correction parameter calculating together. 1.3 satellite clock error
Satellite location is a function of time, therefore, GPS's observations to precision time measurement based on the location that corresponds with the satellite ephemeris information is encoded information via satellite signals transmitted to the receiver. In GPS positioning, whether it is code phase or carrier phase observations, we must ensure strict satellite clock and receiver clock synchronization. In fact, despite the GPS satellites are equipped with high-precision atomic clocks, but they are with the actual GPS time, there are still difficult to avoid bias. The total amount of this bias is about 1 ms or less, for the satellite clock bias, the master station by the satellite, the satellite clock running on the continuous monitoring to determine and provide navigation data via satellite to the receiver. The clock error corrected to ensure synchronization between the satellites is poor, that is maintained at less than 20ns. 1.4More than path error
Multipath error is the GPS receiver in addition receive signals directly from satellites also receive objects from the receiver antenna reflected signal, after
superposition of two signals, GPS signal phase change, resulting in the measurement error. Multipath errors will be generated for the pseudo-range measurement error of several meters, for the carrier phase measurement errors can produce a few centimeters. Multi-path error was not much, but the multipath errors with each station's environment-related, and no rules to follow. Therefore, it is the
high-precision GPS measurement of one of the major sources of error. To reduce the impact of multipath effects, the selection point, the point should try to avoid, such as water, the flat smooth surface and flat buildings, such as reflective materials, the choice of multi-path effects can weaken the antenna, the use of choke coil, as long as possible Each point of observation time, the best time of the day observing the different, in addition to multi-path effects can prevent the antenna. 1.5 Orbital Errors
Satellite orbit error is a measure of the satellite position and the actual satellite position bias. Satellite in motion a variety of perturbation due to the impact of the satellite's orbit is extremely complex and precise determination of a variety perturbation, accurate prediction of satellite orbit is very difficult. At present,
calculated by the satellite broadcast ephemeris position accuracy can only reach 5 ~ 10 m. But the satellite orbit errors can be effectively reduced difference calculation, the differential calculation of satellite position error after the baseline length error △ s △ b and the relationship can be roughly simplified as: △ b = b △ sρ,
Where, b is baseline length; ρ the distance from the satellite to the receiver.
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