SurvNET Introduction

Key Features of SurvNET

• 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. NOTE: 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 pre-defined 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 into a variety of file types including the Carlson (.CRD) file, C&G (.CRD) file, Simplicity file (.ZAK), Carlson SQLITE (.CRDB), or an Autodesk Land Desktop (.MDB) file types. 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 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 a variety of geoid models. 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.

General Rules for Collecting Data for Use in Least Squares Adjustments

Least squares is very flexible in terms of how the survey data needs to be collected. Generally speaking, any combination of angles, and distances combined with a minimal amount of control points and/or azimuths are needed. This data can be collected in any order. There needs to be at least some redundancy in the measurements. Redundant measurements are measurements that are in excess of the minimum number of measurements required to determine the unknown coordinates. Redundancy can be created by including multiple GPS and other control points within a network or traverse. Measuring angles and distances to points in the network that have been located from another setup in the survey creates redundancy. Running additional cut-off traverses or additional traverses to existing control points creates redundancy. Following are some general rules and tips in collecting data for least squares reduction:

• 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. Control is defined as known points or azimuths. They can be held FIXED or allowed to move a designated amount. The minimum amount of control is either two points or one point and a reference azimuth. You can however have as many control points as you wish. 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 data file. The control points and azimuths can be associated with any point in the network or traverse. The control points do 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 issues. Using the same point number twice for different points, not using the same point number when measuring the same point, misnumbering backsights or foresights, and misnumbering control points are all common problems.
• 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 (RW5) file. A big source of problems with new users is a misunderstanding in defining their control for a project.
• Some data collectors 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 been 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.

2D/1D Model

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 to 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. 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 2D/1D model, it is allowed to mix 2D and 3D measurements. Elevations will be calculated 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 an 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/SurvPC 2.0 or higher allows you to mix 2D and 3D data by inserting a 2D or 3D comment record into the .RW5 file. 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. The "Adjust Elevations" box in the project settings must be checked to adjust the calculated elevations. If it is unchecked, elevations will still be calculated if the 3D data is available, but they will not be adjusted.

3D Model

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 the instrument(s). 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.

SurvNET can also automatically reduce field measurements to State Plane coordinates in either the NAD 83 or NAD 27 coordinate systems. If a grid coordinate system is selected, the grid scale factor is computed for each individual line during the reduction. The elevation factor is also 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 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.

Output Coordinate File Types

Coordinates can be written to the following formats, including:

• C&G numeric (*.crd)
• C&G alphanumeric (*.cgc)
• Carlson numeric (*.crd)
• Carlson alphanumeric (*.crd)
• Carlson SQLite (*.crdb)
• MS Access Database (Autodesk Land Desktop) (*.mdb)
• Simplicity (*.zak)
• ASCII P,N,E,Z,D,C (*.nez)

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.

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