基准，但是在测量上，这个参考基准一般使用的是平均海平面（MSL）。MSL被赋予一个0.000英尺或0.000米的高程，地球上所有其它点可以用高于或低于0的高程来描述。高程精确测出的永久点（水准点）被用于大多数区域的测量工作.在中国，利用青岛验潮站从1950年到1956年7年的观测数据处理】和平差，建立了56黄海高程系统。1987年，在依照了验潮站1952到1979年的观测资料后，这个基准被进一步精确——反映长时期海潮变化的85国家高程基准建立起来。虽然，严格说来，国家高程基准在特殊的点上与MSL并不恰好吻合，术语MSL一般还是用来描述它。MSL高程的赋值为0.000英尺或米，高程的差异【高差】可以由下列方法测得：
1.水准测量，直接测得垂直距离，水准测量是高程测量方法中精度最高、使用最普遍的方法
2.三角高程测量，利用测量竖直角和水平或斜距来测高程
3.视距高程测量，利用视距测量，使用工程经纬仪和水准尺；平板仪和照准仪和水准尺；或者自处理视距仪和水准尺测得垂直距离
4.气压水准测量，通过使用气压计测量不同站点大气压力的差值来测高程
5.重力水准测量，通过使用重力计测量不同站点的重力值差值来测高程，用于大地测量学的目的
6.惯性定位系统，含有一个惯性平台，具有三个互相垂直轴，其中一个是“向上”的，所以这个系统产生的输出其中一个就是高程。各自地，据相关报告，在60和100km的距离上，其精度能达到15到50cm，这种装置成本极高，只限于非常大的项目，这些项目地质、气象、授时、以及施特殊限制在传统方法上
7. GPS高程测量，它的参考面是地球椭球面，但是如果在测区有充分的高程点，可以修正至高程基准上来，在这种情况下，其高差的标准差能够达到0.053到0.094米。 水准测量
精度最高、使用最普遍的高程测量方法就是直接测垂直距离的水准测量方法。微差水准测量是利用测量者的水准仪和有刻度的尺来测定远距离的相隔点的高差。
例如，确定欲测关于点A的点B的高程，（如图1），A点的高程已知（BM点），在A和B点之间的中点处安置水准尺，分别以a和b代表在这两处水准尺上的读数。那么，仪器整平后的视线高程就是：HA + a。B点的高程可以由方程来确定
HB=HA + a － b
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除确定B点的高程之外，其它点的高程，低于视线的和水准仪可以看见的点，都可以以相似的方法得到。但是上面的一些术语需要提一下，a 被称为后尺读数，是一个放在已知高程点上的尺的读数，用来求得仪器视线的高程。b 被称为前尺读数，是一个放在转点、水准点、或者是临时水准点之上的尺的读数，用来确定该点的高程。HA + a 指的是仪器高度（HI），是过水准仪的视线的高程。由于大气折光的缘故，实际上视线是有些弯曲的，曲率和折光的影响可以被当作可忽略的值，不必加入球气改正，如果在实际工作中后视距和前视距是相等的。 三角高程测量
三角高程测量适用于困难地形，例如在山区，不能使用常规的微差水准测量。现代的三角高程测量方法是测量到未知点的斜距和垂直角，斜距由电磁波测距仪测得，垂直角（或天顶距）由经纬仪测得，或者利用整合了这两种仪器为一体的全站仪来测。全站仪包含了内置的微处理器，用来根据测得的斜距和垂直角计算和显示水平距离，这种后来的设备导致了三角高程测量被广泛用于多种高度测量工作，包括测绘等高线，三角高程测量的基本原理可以看图2。
当我们用α和S分别表示垂直角和水平距离时，A点和B点之间的高差为：
hAB=S3tanα+i – v
i 是A点上仪器中心的高度，v是B点上目标中心的垂直高度，垂直角为仰角时为正，俯角时为负天顶距总是正的，但是自然的当超过了90°时，它们将产生一个相反的结果。普通精度要求下，三角高程测量方法测高差水平距离不能超过300m，如果要求高的精度，则要相应缩短距离。因为超过300m时，地球曲率和折光影响必需考虑为了消除地球曲率和折光改正的不确定因素，垂直角观测时应采用在观测方向两端尽量同时相向观测的方法。这种观测称为垂直角对向观测。线两端正确的高程之差是计算得到的两高程值的平均值，不管计算有无考虑球气差.这里需要注意的是，测量者在水准测量工作中使用的是参考地球“平均”表面的正高，这个平均表面描述为MSL。然而，GPS方法给出的是地球椭球面到地面站的大地高。
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Methods of Elevation Determination
An elevation is a vertical distance above or below a reference datum.Although vertical distance can be referenced to any datum, in surveying, the reference datum that is universally employed is that of mean sea level (MSL). MSL is assigned a vertical value (elevation) of 0.000 ft or 0.000 m. All other points on the earth can be described by the elevations above or below zero.
Permanent points whose elevations have been precisely determined (benchmarks) are available in most areas for survey use. In China, 7 years of observations at tidal stations in Qingdao from 1950 to 1956 were reduced and adjusted to provide the Huanghai vertical datum
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of 1956. In the 1987, this datum was further refined to reflect long periodical ocean tide change to provide a new national vertical datum of 1985, according to the observations at tidal stations from 1952 to 1979. Although, strictly speaking, the national vertical datum may not precisely agree with the MSL at specific points on the earth’s surface, the term MSL is generally used to describe the datum.MSL is assigned a vertical value (elevation) of 0.000 ft or 0.000 m.Difference in elevation may be measured by the following methods:
1. Direct or spirit leveling, by measuring vertical distances directly. Direct leveling is most precise method of determining elevations and the one commonly used.
2. Indirect or trigonometric leveling, by measuring vertical angles and horizontal or slope distances.
3. Stadia leveling, in which vertical distances are determined by tacheometry using engineer’s transit and level rod; plane-table and alidade and level rod; or self-reducing tacheometer and level rod.
4. Barometric leveling, by measuring the differences in atmospheric pressure at various stations by means of a barometer.
5. Gravimetric leveling, by measuring the differences in gravity at various stations by means of a gravimeter for geodetic purposes.
6. Inertial positioning system, in which an inertial platform has tree mutually perpendicular axes, one of which is “up”, so that the system yields elevation as one of the outputs.Vertical accuracies from 15 to 50 cm in distances of 60 and 100 km, respectively, have been reported.The equipment cost is extremely high and applications are restricted to very large projects where terrain, weather, time, and access impose special constraints on traditional methods.
7. GPS survey elevations are referenced to the ellipsoid but can be corrected to the datum if a sufficient number of points with datum elevations are located in the region surveyed. Standard deviations in elevation differences of 0.053 to 0.094 m are possible under these conditions. Spirit leveling
The most precise method of determining elevations and most commonly use method is direct leveling or spirit leveling which means measuring the vertical distance directly. Differential
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leveling is used to determine differences in elevation between points that are remote from each other by using a surveyor’s level together with a graduated measuring rod. For example, to determine the elevations of desired point B with respect to a point of known elevation A (see Figure 1), the elevation of which (BM) is known to be above sea level, the level is set up at intermediate point between A and B, and rod readings are taken at both locations as a and b respectively. Then the elevation of the line of sight of the instrument (being horizontal) is known to be the line of sight of the instrument HA + a. The elevation of point B can be determined by equation
HB=HA + a － b
In addition to determining the elevation of point B, the elevations of any other points, lower than the line of sight and visible from the level, can be determined in a similar manner. But some terms should be mentioned from above. a is called Backsight (BS) which is a rod reading taken on a point of known elevation in order to establish the elevation of the instrument line of sight. b is called Foresight (FS) which is a rod reading taken on a turning point, benchmark, or temporary benchmark in order to determine its elevation. HA + a refers to the Height of Instrument (HI) which is the elevation of the line of sight through the level. Owing to refraction, actually the line of sight is slightly curved, the effects of curvature and refraction for the horizontal distance can be reduced to a negligible amount and no correction for curvature and refraction is necessary if backsight and foresight distances are balanced in practical operation. Trigonometric Leveling
Trigonometric leveling is used where difficult terrain, such as mountainous areas, precludes the use of conventional differential leveling. The modern approach is to measure the slope distance and vertical angle to the point in question. Slope distance is measured using electromagnetic distance measurers and the vertical (or zenith) angle using a theodolite, or the total station that integrate these two instruments into a single instrument. Total stations contain built-in microprocessors that calculate and display the horizontal distance from the measured slope distance and vertical height. This latter facility has resulted in trigonometrical leveling being used for a wide variety of heighting procedures, including contouring. The basic concept
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of trigonometrical leveling can be seen from Figure 2. When measuring the vertical angle α and the horizontal distance S is used, then the difference in elevation hAB between ground points A and B is therefore:
hAB=S×tanα+i – v
where i is the vertical height of the measuring center of the instrument above A and v is the vertical height of the center of the target above B. The vertical angles are positive for angles of elevation and negative for angles of depression. The zenith angles are always positive, but naturally when greater than 90° they will produce a negative result. Trigonometrical leveling method of determining difference in elevation is limited to horizontal distance less than 300 m when moderate precision is sufficient, and to proportionately shorter distances as high precision is desired. For the distance beyond 300 m the effects of curvature and refraction must be considered and applied. To eliminate the uncertainty in the curvature and refraction correction, vertical-angle observations are made at both ends of the line as close in point of time as possible. This pair of observations is termed reciprocal vertical-angle observation. The correct difference in elevation between the two ends of the line is the mean of the two values computed both ways either with or without taking into account curvature and refraction.The important notes should be mentioned here is that surveyors used to working with spirit levels have referenced orthometric heights (H) to the “average” surface of the earth, as depicted by MSL. However, the elevation coordinate (h) given by GPS solutions refers to the height from the surface of the ellipsoid to the ground station.
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