n4ce works with different data types, but to display data graphically it needs to be in vector, or X, Y & Z format. We assume data falls on a plane surface and in the case of angular measurements needs to be reduced to coordinates.
The Reduction process requires control data. A folder called Stations is provided for this purpose. As part of these calculations, corrections can be made for local Scale Factor, Curvature & Refraction and Mean Sea Level. These can be found in the Settings à Survey Corrections menu.
Reduction is accessed from the right click menu after focusing on the Project Tree Observations folder you wish to reduce. Control measurements appear in bright yellow with a -99 or STN point number.
Selecting Reduction from the Right Click Menu
Free Station Calculation
This is also known as Resection and involves measuring angles and distances from an occupied station which is unknown (in terms of coordinates) to fixed stations (whose coordinates are known). The process will calculate the coordinates of the occupied station which can then be used in reduction of observed detail from this point.
Known Control – Stations
In the example shown here we have an instrument setup at Station A, which is unknown. The fixed or known control points are stored beforehand in the Stations folder, shown above. These are I5, I2 and I3.
Observed Control Measurements
You may note that there are two sets of observations to stations I5 and I2. One of these is on FL the other FR. n4ce will mean multiple FL/FR readings provided that the Target Heights are the same for each reading.
Free Station (Resection)
Right clicking over station A on the Project Tree and selecting Free Station results in the dialog box as shown above.
The ticks alongside the Code column indicate the stations that are used in calculations. If you double click over a station the tick will change to a cross I2 and this station will be removed from the calculation.
Note that you may force a Scale and produce a Report. Always check the summary of Std Errors.
The process uses both angles and distances are considered in a least squares Helmhert calculation. First an approximation is made for the free station coordinates which are used to calculate coordinates of the known stations using measured angles and distances. These approximations are adjusted using the Helmhert calculation.
Free Station Calculation Report
Traverses are used to transfer “control” from one point to another. This implies measurements to and from several station setups. Four different types of traverse are recognised, namely:
- Closed (Loop) – starts and finishes at the same point, which must be know. The orientation is taken from a fixed (typed) bearing between first and last points on the traverse. The reading to this point is initially taken.
- Control - starts and finishes at fixed control points with opening and closing bearings.
- Pipe – like the Control traverse but has no closing bearing. ie the last point on the traverse is not occupied.
- Open – like a Pipe traverse, but does not observe to a known point for closure. No adjustments for misclosure can therefore be made, but BS and FS readings to and from the same points will be meaned.
- GPS Link – this is a separate option and is used to traverse between two known points, which are not occupied. The traverse is simply rotated in the direction of the two points and scaled to fit.
In all cases “known” or fixed points must be identified. Both angles and distances are measured from these “fixed” to “unknown” points. The purpose of traverse adjustment is to calculate coordinates of the unknown points.
If you start at a known point and calculate the coordinates of points along the traverse using measured angles and distances to another known point this will create a “closing error”. Another purpose of these calculations is to distribute these misclosures, or small measuring errors, throughout the traverse. Bowditch and Transit adjustment methods are supported. Corrections for Scale Factor, Curvature & Refraction and Mean Sea Level can be applied.
In the example shown above, the traverse starts at STN22 and finishes at STN20. Angles and distances are measured between each station. Since we know the coordinates of fixed points a back bearing and forward bearing can be determined.
It doesn’t matter if you measure horizontal angles clockwise or anticlockwise, provided you are consistent. Tools are available in the Traverse Wizard to switch BS and FS readings to make Horizontal angles wholly clockwise or anticlockwise.
Multiple Setups - Traverse Measurements
Before we look at the Traverse Wizard, it’s important to know how n4ce treats observations.
- Setups must appear under the same job folder heading, as shown above AIC_TRAV, which is taken from sample data in CPD2.LOG in your training folder. If you carry out a traverse over a number of days, you may wish to Append or Copy data into an existing job folder.
- All the control measurements are identified with a -99 (STN) point number and can be mixed with observations to other measured detail. Control measurements are highlighted in bright yellow.
- The first reading in the list must be your Backsight (BS). The last control reading is assumed to be a Foresight (FS). Any control side-shots (SS) in between these readings will be ignored. You will have the option to swap your BS and FS or introduce SS as either a BS or FS.
- Where multiple readings to the same station are found, using Face Left (FL) and Face Right (FR) observations, the first occurrence of a reading to a control point will be assumed to be FL. Any subsequent reading will be compared (adjusted) to this face. This allows for the traditional FL clockwise readings followed by FR anticlockwise readings.
- Multiple control readings will be meaned, considering comments made in 4. above.
- If you measure “to” (FS) and “from” (BS) distances and vertical angles between two stations, the horizontal and vertical distances will be meaned. A report summerises all the extracted and meaned data used in traverse calculations.
The Traverse Wizard
It’s important that you are aware of where Station data is stored and how it’s used.
During Reduction the known coordinates of each instrument setup and RO to other known stations are used to calculate the coordinates of detail observations. The Comma Code ,I is used to ignore unwanted control readings.
Stations can be typed into the Stations folder, imported from external files, copied from another project (and from coordinates in a Model or Co-ordinates folders) or calculated from detailed measurements. Stations will be given a status depending upon how they were created.
- Known – typed or imported from external files
- Adjusted – processed as part of free station or traverse calculations
- Derived – calculated as part of a reduction process and not included in traverse adjustments
- Unknown – as the name implies these have yet to be assigned coordinates
It is assumed that only one coordinate system will be used in a Project. If a Station appears more that once in the Stations folder only the FIRST occurrence of a Station will be used in calculations.
Right click over the folder containing the Traverse setups and select the Traverse menu option as shown here.
Stage 1 extracts control measurements using the rules already identified and displays them in a list. The BS and FS stations have been identified with a red and blue triangle and can be changed using the buttons in the centre panel.
The Traverse Wizard can handle multiple traverses, but only one at a time. The >< buttons between the two windows allow you to move stations from left to right windows, identifying individual traverse observations.
The Automatic button will thread a traverse between Setups, BS and FS readings until it can go no further.
Stopping criteria include missing a match the last FS or finding a station which is known. A known station is one that appears in the Stations folder or in the case of a closed traverse, the last FS points to the first instrument setup.
The Wizard will automatically identify the type of traverse you are using from the available data provided. In this example a Closed traverse.
Stage 2 involves calculating the approximate coordinates and misclosure. After pressing the Next button, the following page will be displayed. This contains the first stage calculations BEFORE adjustment.
Pre-Adjusted Table with Misclosures
The first place to visit is the Misclosures. This gives an indication of goodness of fit of your traverse. The linear fractional error (LF Error) is the total length of the traverse divided by the lack of fit and should be at least 1 in 50,000.
If you were working with a Control or Pipe traverse the Finish coordinates would also be shown. You can change these if necessary, at this stage, and the displayed coordinates will change accordingly.
In the case of a Closed traverse the Bearing is the value between the first and last points on the traverse.
Stage 3 will calculate the final adjusted coordinates of traverse stations and is the penultimate stage of the traverse adjustment. There are two choices of adjustment.
- Bowditch – adjusts misclosures on basis of length of each traverse leg.
- Transit – adjusts misclosure on basis of eastings and northings projection of each leg. This keeps the bearing of each leg fixed.
In both cases the angular misclosure is distributed equally between each setup and the misclosures calculated again. These misclosures are then distributed according to either Bowditch or Transit rules.
Final Adjusted Coordinates
After pressing Finish the process enters Reduction, which uses the adjusted coordinates to calculate observational detail. A report can also be presented displaying details of the calculations.
In the final Reduction phase, a list of Adjusted or Derived coordinates will be displayed with a comparison between measured and adjusted values. Tolerance checking is performed in the RO Review and is a must for viewing. Step down each Station setup using the up/down arrow keys.
During reduction only the first occurrence of a station will be used, if multiples are found in the Stations folder. If you have observed your BS more than once an average will be taken unless the Ave Back Sights check box is un-ticked.
A full Report will be presented, as shown below, showing how control observations were meaned, and RO Review.
Traverse and Reduction Report
This is a special traverse option introduced to allow a control survey to link between two points, generally fixed using GPS (GNNS). The traverse links between these stations but does not occupy them. An example is where you may traverse through a wood tying into GPS points at either end. The option can be found just below the Traverse option when you right click over the survey data folder. See above.
GPS Link Survey
Since survey data making up the traverse is measured on a plane and GPS is to a projection it is very unlikely that the two will match. You will have to apply a scale correction.
The process starts by calculating the approximate coordinates of the end from the start station using an arbitrary start bearing and traverse measurements. After rotating the traverse in line with the end point, the difference between the calculated end and the known coordinates is the misclosure.
By scaling up/down the traverse it can be made to “fit” the end. This scaling takes place in all directions. In essence a two-point transformation, with a scale. Whilst this is strictly not an adjustment process it will provide a correction to all lengths in the traverse. Angles remain the same.
Stage 1 After selecting the GPS Link option from the right click menu, the start point is identified, which in this case is point 1 and not listed as a setup station.
GPS Link Selection
The traverse is forward calculated until it finds the next known point, which in this case is point 9, again not occupied. A dialog of the points found and misclosures calculated will then be displayed.
The misclosure is the distance adjustment required to force a fit. The traverse will be scaled to fit the start and finish points. The LF Error is the total length of the traverse divided by the misclosure and should fall between 50,000 and 100,000 if proper traverse procedures are adopted, where the target and instrument are leap frogged between setups.
GPS Link Summary
This is not a wizard so very few options are available. The reporting will summerise the results in the previous display.
Stage 2 After pressing the OK button the corrected coordinate of points on the traverse will be added to the Stations menu and you will enter the Reduction dialog, as previously shown with normal traverse adjustments.
The RO Review gives you an indication of the goodness of fit for the survey, using tolerances provided. The exclamation marks and subscript numbers provide the number of tolerance failures. A reduction report follows once you select OK to continue.
Reduction with RO Review Checking