TPC can compute the position of a lost corner using the Single Proportionate Measurement method as described in the Manual of Surveying Instructions 2009, page 168, section 7-16. This method requires two existing corners, record data from the existing corners to the lost corner and retracement data from the existing corners to the lost corner. This method uses grid bearings and distances with the option to compute the latitudinal offset.
TPC computes the inversed retracement and record distances between the two existing corners and applies the ratio (retrace distance / record distance) to the record distances. It then uses the inversed retracement bearing/azimuth to compute the position of the lost corner.
Single Proportion (5:35) - Do single proportions with or without offsets to the latitudinal arc (East-West lines) ! See how TPC batches your data in traverses.
Storing Retrace and Record data in separate traverses is the preferred method. TPC calls this 'batching' data. The name of the Retrace traverse must include any of the keywords Retrace, Measure, Existing, Temporary. The name of the Record traverse must include the key word Record.
Formatting the Batch Traverses
The batch traverses should include data as follows:
f (existing corner) bearing / distance e (temporary or lost corner) bearing / distance d (existing corner)
You will typically open the Single Proportion Dialog from the Traverse View, where you will select the temporary (Retrace) or lost (Record) point in the Traverse View, then choose COGO | Single Proportion (PLSS).
Formatting the Record Traverses with Intermediate Calls
You can include any number of record calls to an existing corner. The trick is to select the two existing corners along with the lost corner before choosing COGO | Single Proportion. TPC uses the proximity of your three selected traverse points to determine the existing corners and the lost corner.
f (existing corner) <- select this point bearing / distance e (temporary or lost corner) <- select this point bearing / distance c (intermediate call) bearing / distance d (existing corner) <- select this point
Single Proportion Report
To report the solution, choose Summary from the dialog.
The latitudinal offset (if requested) is computed using the following equation from Elgin-Knowels-Sinne as set out in POB tech tips. If you have not selected a coordinate reference system for this survey, the offset will not be computed.
offset = (0.6668) (LW) (LE) (Tan Latitude)
where LW = distance from the lost corner to the west existing corner and LE = distance from the lost corner to the east existing corner.
The latitude is computed based on the coordinate reference system selected for the survey and the coordinates of the computed position of the lost corner before it is offset.
Saving a Computed Position
As you work between the Single Proportion dialog box and the summary, you haven’t actually created or updated any points. You have simply been looking at the data that is being calculated. You never actually store any point data until you choose Save to tell Traverse PC to either update an existing point or save a new point.
To save the position you just computed
•Enter a new or existing survey point label in the Computed Position of Lost Corner field.
•Answer any prompts about TPC displays about the point label you entered. This will be your opportunity to update the position of an existing survey point.
•Choose Save. You have now updated the survey to include the computed position for the lost corner.
