Introduction

• SurvNet reduces survey field measurements to coordinates in assumed, UTM, SPC83 SPC27, and a variety of other coordinate systems. SurvNET calculates the minimum necessary corrections to measured horizontal angles, slope distances and vertical angles in order to fit the desired control. SurvNET can only process raw field measurements, it is not designed to process bearing or azimuth traverses. If you wish to use SurvNET to process your traverses, you must collect the angles and distances.
  
• In the 2D/1D model in a state plane coordinate system, a grid factor is computed for each individual line during the reduction. The elevation factor is computed for each individual line if there is sufficient elevation data. If the raw data has only 2D data, the user has the option of defining a project elevation to be used to compute the elevation factor.

• SurvNet  supports a variety of  map projections and coordinate systems including  the New Brunswick Survey Control coordinate system, UTM, and user defined systems consisting of either a predefined ellipsoid or a user defined ellipsoid and one of the following projections, Transverse Mercator, 1 Standard Parallel Lambert Conformal, 2 Standard Parallel Lambert Conformal, Oblique Mercator, and the Double Stereographic projection.

• A full statistical report containing the results of the least squares adjustment is produced and written to the report (.RPT) file. An error report (.ERR) file is created and contains any error messages that are generated during the adjustment.

• Coordinates can be stored in a Carlson (.CRD) file, C&G (.CRD) file, Simplicity file (.ZAK) or an LDD file. An ASCII coordinate (.NEZ) file is always created that can be imported into most any mapping/surveying/GIS program. The user has the option to compute unadjusted preliminary coordinates.

• There is an option to compute traverse closures during the preprocessing of the raw data. Closures can be computed for both GPS loops and total station traverses. Closure for multiple traverse loops in the same raw file can be computed.

• When processing Angle-Only records for triangulation, if there is a zenith angle and rod height (zero is a valid rod height), a 3D triangulation will be performed, calculating an elevation of the triangulation point. This is true in both the 3D and 2D/1D models.

• SurvNet can combine GPS vectors and total station data in a single adjustment. GPS Vector files from Leica, Thales, Topcon and Trimble can be input, as well as GPS files in the StarNet format. Additionally GPS vectors can be read from NGS G-files. There is also an option to read the G-file section of an Opus report.
  

• SurvNet includes a variety of blunder detection routines. One blunder detection method is effective in detecting if the same point number has been used for two different points. Additionally this blunder detection method is effective in detecting if two different point numbers have been used for the same physical position. This method also flags other raw data problems. Another blunder detection method included in SurvNet is effective in isolating a single blunder, distance or angle in a network. This method does not require that there be a lot of redundancy, but is effective if there is only one blunder in the data set. Additionally, SurvNet includes a blunder detection method that can isolate multiple blunders, distances or angles in a network. This method does require that there be a lot of redundancy in the network to effectively isolate the multiple blunders.

• Other key features include: Differential and Trig level networks and loops can be adjusted using the network least squares program. Geoid modeling is used in SurvNet, allowing the users to choose between the Geoid99 and the Geoid03 model. The user can alternately enter the project geoid separation.  There are description codes to identify duplicate points with different point numbers. The user can specify the confidence interval from 50 to 99 percent.

Network Least Squares Reduction

SurvNet  performs a least squares adjustment and statistical analysis of a network of raw survey field data, including total station measurements, differential level data and GPS vectors. SurvNet simultaneously adjusts a network of interconnected traverses with any amount of redundancy. The raw data can contain any combination of angle and distance measurements, and GPS vectors. SurvNet can adjust any combination of trilaterations, traverses, triangluations, networks and resections.  The raw data does not need to be in a linear format, and individual traverses do not have to be defined using any special codes. All measurements are used in the adjustment.

SurvNet implements the standard parametric observation equation method with independent weighting for azimuths, directions, angles, distances, GPS baselines, coordinates, elevations and level data to compute least squares estimates of all unknowns in accordance with well-established reference texts such as Adjustment Computations: Spatial Data Analysis (4th Edition) Paul R. Wolf, Charles D. Ghilani.

General Rules for Collecting Data for Use in Least Squares Adjustments

• Backsights should be to point numbers. Some data collectors allow the user to backsight an azimuth not associated with a point number. SurvNet requires that all backsights be associated with a point number.
• There has to be at least a minimum amount of control. There has to be at least one control point. Additionally there needs to be either one additional control point or a reference azimuth. Control points can be entered in either the raw data file or there can be a supplemental control point file containing the control point. Reference azimuths are entered in the raw data file. The control points and reference azimuths do not need to be for the first points in the raw file. The control points and azimuths can be associated with any point in the network or traverse. The control does not need to be adjacent to each other. It is permissible, though unusual, to have one control point on one side of the project and a reference azimuth on the other side of the project.
• Some data collectors do not allow the surveyor to shoot the same point twice using the same point number. SurvNet requires that all measurements to the same point use a single point number. The raw data may need to be edited after it has been downloaded to the office computer to insure that points are numbered correctly. An alternative to renumbering the points in the raw data file is to use the 'Pt Number substitution string' feature in the project 'Settings' screen. See the 'Redundant Measurement' section for more details on this feature.
• The majority of all problems in processing raw data are related to point numbering problems. Using the same point number twice to different points, not using the same point number when shooting the same point, misnumbering backsights or foresights, and misnumbering control points are all common problems.
• A big source of problems with new users is a misunderstanding in defining their control for a project. It is always best to explicitly define the control for the project. A good method is to put all the control for a project into a separate raw file.
  
• Some raw data collector files may have preliminary unadjusted coordinates included with the raw data. These coordinate records should be removed from the raw file. The only coordinate values that should be in the raw file are the control points. Since there is no concept of 'starting coordinates' in least squares there is no way for SurvNet to determine which points are considered control and which points are preliminary unadjusted points. So all coordinates found in a raw data file will be considered control points.
• When a large project is not processing correctly, it is often useful to divide the project into several raw data files and debug and process each file separately as it is easier to debug small projects. Once the smaller projects are processing separately they can be combined for a final combined adjustment.
  

Two Mathematical Models, 2D/1D and 3D

SurvNet gives the user the option to choose one of two mathematical model options when adjusting raw data, the 3D model and the 2D/1D model.

In the process of developing SurvNet numerous projects have be adjusted using both the 2D/1D model and the 3D model. There are slight differences in final adjusted coordinates when comparing the results from the same network using the two models. But in all cases the differences in the results are typically less than the accuracy of measurements used in the project. The main difference in terms of collecting raw data for the two different models is that the 3D model requires that rod heights and instrument heights need to be measured, and there needs to be sufficient elevation control to compute elevations for ALL points in the survey. When collecting data for the 2D/1D model the field crews do not need to collect rod heights and instrument heights.

In the 2D/1D model raw distance measurements are first reduced to horizontal distances and then optionally to grid distances.  Then a two dimensional horizontal least squares adjustment is performed on these reduced horizontal distance measurements and horizontal angles. After the horizontal adjustment is performed an optional one-dimensional vertical least squares adjustment is performed in order to adjust the elevations if there is sufficient data to compute elevations. The 2D/1D model is the model that has been traditionally been used in the past by non-geodetic surveyors in the reduction of field data. There are several advantages of SurvNet's implementation of the 2D/lD model. One advantage is that an assumed coordinate system can be used. It is not necessary to know geodetic positions for control points. Another advantage is that 3D raw data is not required. It is not necessary to record rod heights and heights of instruments. The 2D/1D model allows you to mix 2D and 3D measurements. Elevations are not required for the control points. The primary disadvantage of SurvNet's implementation of the 2D/1D model is that GPS vector data cannot be used in 2D/1D projects. 

In the 3D model raw data is not reduced to a horizontal plane prior to the least squares adjustment. The 3 dimensional data is adjusted in a single least squares process. In SurvNet's implementation of the 3D model XYZ geodetic positions are required for control. The raw data must contain full 3D data including rod heights and measured heights of instrument. The user must designate a supported geodetic coordinate system. The main advantage of using the 3D model is that GPS vectors can be incorporated into the adjustment. Another advantage of the 3D model is the ability to compute and adjust 3D points that only have horizontal and vertical angles measured to the point. This feature can be used in the collection of points where a prism cannot be used, such as a power line survey. 

When using the 2D/1D model if you have 'Vertical Adjustment turned' ON in the project settings, elevations will be calculated and adjusted only if there is enough information in the raw data file to do so. Least squares adjustment is used for elevation adjustment as well as the horizontal adjustment. To compute an elevation for the point the instrument record must have a HI, and the foresight record must have a rod height, slope distance and vertical angle. If working with .CGR raw data a 0.0 (zero) HI or rod height is valid. It is only when the field is blank that the record will be considered a 2D measurement. Carlson SurvCE 2.0 or higher allows you to mix 2D and 3D data by checking or unchecking the 3D MODE checkbox in the Configuration dialog (General Tab). A comment record "--Elevation: 3D" or "--Elevation: 2D" will be inserted into the .RW5 file and SurvNET will pay attention to those records. A 3D traverse must also have adequate elevation control in order to process the elevations. Elevation control can be obtained from the supplemental control file, coordinate records in the raw data file, or elevation records in the raw data file.

SurvNet can also automatically reduce field measurements to state plane coordinates in either the NAD 83 or NAD 27 or other supported geodetic coordinate systems. In the 2D/1D model a grid factor is computed for each individual line during the reduction. The elevation factor is computed for each individual line if there is sufficient elevation data. If the raw data has only 2D data, the user has the option of defining a project elevation to be used to compute the elevation factor.

A file with the extension .OUT is always created and contains an ASCII formatted coordinate list of the final adjusted coordinates formatted suitable for printing. Additionally an ASCII file with an extension of .NEZ containing the final adjusted coordinates in a format suitable for input into 3rd party software that is capable of inputting an ASCII coordinate file..

SurvNet produces a wealth of statistical information that allows an effective way to evaluate the quality of survey measurements. In addition to the least squares statistical information there is an option to compute traverse closures during the preprocessing of the raw data. Traverse closures can be computed for both GPS loops and total station traverses. This option has no effect on the computation of final least squares adjusted coordinates. This option is useful for surveyors who due to statutory requirements are still required to compute traverse closures and for those surveyors who still like to view traverse closures prior to the least squares adjustment.