5.5　Establishment of GPS Main Control Network and GPS Field Control of Aerial Survey

5.5.1　The GPS main control network shall be arranged according to the actual needs and traffic in the survey area.The distance between main control points should not be more than 10km.

Permanent stakes shall be buried at the control points of main control network.

5.5.2　For the survey precision of GPS main control network，the relative RMSE of the weakest side shall not exceed 1/20，000，while the chord length between neighboring points and the precision of geodetic elevation difference shall satisfy the following requirements:

1　The precision of chord length between neighboring points of the GPS main control network shall be calculated using the formula below（5.5.2），and shall satisfy the requirements on precision of chord length between points stipulated in Table 5.5.2-1.

Where:

σ——standard difference，mm.

a——fixed error，mm.

b——scale error coefficient（10-6）.

d——distance between neighboring points，km.

2　The precision of geodetic elevation difference surveyed with GPS main control network shall be calculated using formula 5.5.2，and shall comply with the requirements on precision of geodetic elevation difference between points stipulated in Table 5.5.2-2.

5.5.3　Connection survey and conversion shall be made between the GPS main control network and national horizontal control network，and the number of connection survey points shall not be less than three.The GPS main control network for the same project may be divided into several projection sections.

5.5.4　Where the GPS main control network is used for connection survey of elevation，the number of vertical points for connection survey shall not be less than three，which shall be evenly distributed and control the whole network.

5.5.5　Obvious direction signs should be provided around the control points of GPS main control network，and the description of GPS points shall be mapped in the field.

5.5.6　The arrangement of photo control points on photographs shall comply with the following provisions:

1　The photo control points with the picture format of 23cm×23cm shall be more than 1.5cm away from the photo edges，and the photo control points with the picture format of 16cm×9cm shall be more than 0.5cm away from the photo edges，and more than 1mm away from any sign on the photo.

2　The photo control points shall be more than 4.5cm away from the heading line.

5.5.7　The photo control points shall be arranged on the flight lines in accordance with the following provisions:

1　The upper and lower control points at the start and end of the flight line should be located in the straight line that passes through the principal point of photograph and is perpendicular to the heading line，and these control points shall deviate from each other by less than half baseline when there is difficulty.The upper and lower pairs of points shall be located within the same stereopair.

2　When the points are arranged for a single flight line，a pair of control points shall be arranged for every five baselines for the picture format of 23cm×23cm，and for at least every eight baselines for the picture format of 16cm×9cm.The number of photo control points for each flight strip shall not be less than six.Common photo control points shall be arranged at the junction of two flight lines.

3　When the longitudinal overlap is less than 56%，points shall be arranged respectively for the areas with the aerial gap being the boundary.The aerial gap shall be surveyed in field.

5.5.8　The point arrangement for a block shall conform to the following provisions:

1　When the block is used to densify horizontal and vertical control points，every five baselines with the picture format of 23cm×23cm shall be provided with one pair of control points，and at least eight baselines with the picture format of 16cm×9cm shall be provided with one pair of control points.The control points should be arranged using the six-point method（see Figure 5.5.8a）or eight-point method（see Figure 5.5.8b），and the number of baselines between each pair of vertical control points along the direction of flight line should be 3-5.

2　Where principal points or standard points are located in water or covered by cloud shadows，shades or snow shadows or other causes lead to fuzzy images，or no obvious surface features exist，the following provisions shall be complied with:

1）As long as the ranges and positions on water do not affect the connection of stereoscopic models，the control points may be arranged as if along normal flight lines；

2）Where no obvious object can be selected within 3cm of the principal point or no connection point can be selected within the longitudinal overlap of three photographs，the stereopair on water shall employ control points arranged in the field.

3　Where lateral standard points are located on water and no connection point can be selected from the longitudinal overlap of three photographs more than 4cm away from the heading line，the stereopair on water shall employ control points arranged in the field.

4　The common photo control points of adjacent flight lines shall be arranged near the centerline of lateral overlap，more than 4.5cm away from the photo heading lines of the upper and lower strip.

5.5.9　The control points shall be dominated by point interpretation and supplemented with pin-points.Point selection scheme should be worked out indoors，and the pinprick of points shall be conducted in the field.

5.5.10　The selection and pinprick of photo control points shall conform to the following provisions:

1　The horizontal control points shall be located at the intersection of fixed surface features that have proper intersection angles and clear images or the center of punctual features that have an image size of less than 0.2mm.

2　The vertical control points shall be at target points whose elevations are not prone to change and the images on adjacent photos are clear.

3　The positions of horizontal and vertical control points shall meet the requirements for pinprick for both horizontal and vertical control purposes.

4　The photo control points shall be selected and pinpricked at the target points that are convenient for GPS station establishment and observation，and the identification precision of the target points in the field shall not exceed 0.1mm.

5.5.11　Pinpricking shall be carried out with the help of a stereoscope or magnifier and the points must be made through.The pinprick error of horizontal and vertical control points shall not be 0.1mm larger than that on photos.Double-pinprick shall be avoided.In addition，the following requirements shall be met:

1　If horizontal & vertical control points are located on a vertical surface feature such as wall，the vertical control points should be located at higher places.

2　If it is difficult to select the target pinprick point，a group of pinpricks should be selected.

5.5.12　The triangulation point，leveling point，traverse point and other monument set points should be pinpricked on the aerial photographs.The point position sketches shall be mapped，and the height annotation marks with respect to the ground shall be rounded up to 1cm.

5.5.13　The decoration of the front of a control photo shall include point mark and number.Common photo control points shared by flight lines shall be transferred on the basic photos of adjacent lines，and the line number and photo number of the pinpricks shall be marked.

5.5.14　To decorate the back of control photos，the positions of control points shall be marked with appropriate symbols，a detailed and accurate sketch of 2cm×2cm should be mapped，and simple text description shall be added to describe the exact position.To describe a horizontal position，with the text head of photo number oriented upwards，use"up"，"down"，"left"and"right"as well as gray scale and color for description.In case of mountainous areas where no obvious surface features exist，the landform sketches of contour or the profiles may be mapped.The personnel who pinprick and check the points shall sign their names and the dates.

5.5.15　Photo control points shall be numbered in a unified way，and the numbers should contain the strip number and sequence number.

5.5.16　The survey of photo control points should be completed simultaneously with that of the control points of GPS main control network.However，the adjustment of GPS main control network shall be conducted separately.The GPS control network，which shall be composed of one or more independent observation loops，shall be formed by means of network connection，side connection or annexed transverses.Free baseline shall be avoided in the network.

5.5.17　When using GPS for survey，the surveyors shall pay constant attention to the satellite signal received by the GPS receiver and information storage，and complete the field observation notebook.In case of any abnormal reception and storage，the surveyors shall make adjustment in time and notify other receivers to adjust the observation plan if necessary.In addition，the following requirements shall be met:

1　For the observation during a period，the surveyors should measure the antenna heights（rounded up to 1mm）respectively before and after observation.The difference between these two heights shall not exceed 3mm，and their average shall be taken as the height of antenna.

2　During the observation，no radio station shall be used within 50m of the receiver，and no interphone or mobile phone shall be used within 10m.

3　When performing survey using GPS in rapid static mode or static mode，the sampling interval of each GPS receiver shall remain the same during the same observation unit.

4　During one observation session，it is not allowed to perform self tests，change the satellite elevation mask angle，the data sampling interval，or the antenna position.

5　The data or documents stored in the receivers shall be transmitted to a computer or external storage device after the observation each day.Field observation data shall be copied in duplicate without any deletion or revision.

5.5.18　The basic technical requirements for GPS main control network survey shall comply with the regulations of Table 5.5.18.

5.5.19　The observation data shall be subject to processing and quality analysis in time once the field survey is finished.During each observation session，the simultaneous observation loop and non-simultaneous observation loop shall be verified for misclosure in accordance with the following requirements:

1　The simultaneous observation of multiple receivers will bring about trilateral simultaneous observation loops.Therefore，after processing the observation values of various legs，the surveyors shall check all possible misclosures of trilateral loops.The misclosure components of coordinate difference of trilateral simultaneous observation loops shall be smaller than the indictors below:

Where:

WX——misclosure of longitudinal coordinates of the trilateral simultaneous observation loop；

WY——misclosure of horizontal coordinates of simultaneous observation loop；

WZ——misclosure of vertical coordinates of simultaneous observation loop；

Ws——linear misclosure of the trilateral simultaneous observation loop；

σ——RMSE of the chord length of the simultaneous observation loop that is calculated with the mean chord length using formula（5.5.2）.

2　Where multiple independent observation legs form a non-simultaneous closed observation loop，the loop shall be verified.The misclosure components of coordinate differences shall be smaller than the indictors below:

Where:

WX——misclosure of the longitudinal coordinates of the non-simultaneous observation loop；

WY——misclosure of the horizontal coordinates of the non-simultaneous closed observation loop；

WZ——misclosure of the vertical coordinates of the non-simultaneous closed observation loop；

Ws——misclosure of the whole length of the non-simultaneous closed observation loop；

n——number of legs comprising the non-simultaneous closed observation loop；

σ——RMSE of chord length of the non-simultaneous closed observation loop that is calculated with the mean chord length using formula（5.5.2）.

5.5.20　For one leg，the discrepancy of results during any two sessions shall be smaller than 2times the nominal precision of GPS receivers.

5.5.21　Where the observation results are found unsatisfactory，comprehensive analysis shall be made on the GPS results and supplementary survey or resurvey shall be carried out on the unsatisfactory data.

5.5.22　Select the initial calculation data according to actual needs and make inspection and analysis on them before making adjustment of the GPS control network.

5.5.23　When processing the data for a survey area，the RMSE of precision for single point positioning coordinates at the selected starting point shall be smaller than 25m.

5.5.24　During the adjustment，unconstrained adjustment shall be first carried out to check the compliance precision of GPS baseline vector network and whether an obvious system error exists between baseline vectors，and delete the baseline legs which contain gross errors.

5.5.25　Three-dimensional or two-dimensional constraint adjustment may be used for the GPS control network，which shall satisfy the following requirements:

1　For constrained adjustment，the coordinates，distance and azimuth of control points may be constrained as fixed values subject to compulsory constraint.

2　When three-dimensional constraint adjustment is used，it is allowed to assume the geodetic elevation of only one point as the original data for elevation calculation.

3　When two-dimensional constraint adjustment is used，the three-dimensional GPS vectors shall be first converted to two-dimensional baseline vectors.

5.5.26　After calculation，all data results processed shall be analyzed to produce results of GPS adjustment.The results of GPS adjustment shall include the GPS survey results，precision evaluation，name and version of adjustment software，model and number of GPS equipment，calculation date，observers，and calculators.

5.5.27　For the field control of GPS main control network and GPS aerial photographing，the following data shall be submitted:

1　Control photos；

2　Observation notebooks of field control points for aerial photographing with GPS main control network；

3　Adjustment results；

4　Reports on GPS field control results.