Key Features of SurvNet
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.
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 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 needed 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 point 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.
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 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 the following formats:
C&G numeric (*.crd)
C&G alphanumeric (*.cgc)
Carlson numeric (*.crd)
Carlson alphanumeric
(*.crd)
Carlson SQLite
(*.crdb)
MS Access Database (LDT) (*.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..
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.