TPC can compute the position of a lost corner using the Double Proportionate Measurement method as described in the Manual of Surveying Instructions 2009, page 168, section 7-16. This method requires two-four known corners, record data from the known corners to the lost corner and retracement data between the known corners. This method uses grid bearings and distances per the manual.
TPC computes the inversed retracement and record latitudes and departures between the two known corners and applies the ratio (retrace lat or dep / record lat or dep) to the record latitudes and departures.
Double Proportion (10:42) - Do double proportions with or without cardinal offsets!
You will typically open the Double Proportion Dialog from the Traverse View, where the selected point becomes the Lost Corner in the dialog.
Batching Data
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:
T (temporary or lost corner) bearing / distance BM SS (side shot to existing corner) bearing / distance CM SS (side shot to existing corner) bearing / distance AM SS (side shot to existing corner) bearing / distance DM SS (side shot to existing corner) bearing / distance
You will typically open the Double Proportion Dialog from the Traverse View, where you will select the temporary (Retrace) or lost (Record) point in the Traverse View, then choose COGO | Double 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 existing corners along with the lost corner before choosing COGO | Double Proportion.
X (lost corner) <- select this point bearing / distance B SS (side shot to existing corner) <- select this point bearing / distance C SS (side shot to existing corner) <- select this point bearing / distance A SS (side shot to existing corner) <- select this point bearing / distance E (foresight to existing corner) bearing / distance D SS (side shot to existing corner) <- select this point bearing / distance
2-Point and 3-Point Solutions
Using the traverse configuration above, you can include either 2 or 3 sideshots after the temporary (Retrace) or lost (Record) corners. TPC will use just those points and switch to the appropriate 2-Point or 3-Point solution.
As you work between the Double 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.
