TPC can compute the position of a lost corner using the Double Proportion method as described in the Manual of Surveying Instructions 2009, page 168, section 7-16. This method requires two three or four known corners, record data from the known corners to the lost corner and retracement data between the known corners.
TPC computes the inversed retracment 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. TPC currently uses grid bearings and distances per the manual.
TPC uses the words Retrace and Measured interchangeably when referring to lost corner computations. These are the measurements you make fromt he existing (found) corners, retracing the footsteps of the original survey. These measurements result in a temporary location for the lost corner, which TPC refers to as the Temporary Corner. And because the temporary corner can be set in the field, TPC provides offset distance and direction data to set the lost corner from the temporary corner.
TPC does NOT use typical batch computations involving formatted text files. Such batch methods are often cumbersome to learn and use.
Instead, TPC expects you to create Retrace and Record traverses for each lost corner. The traverses are a permanent part of the survey and can be re-used as needed to recompute the lost corners. In addition, the traverses provide drawings and printouts of the data you used to compute the lost corners.
We recommand that you open the Double Proportion Dialog from the Traverse View using traverse layouts like the printout below. The words 'Retrace' in the first traverse name and 'Record' in the second traverse name identify which data in the Double Proportion dialog is represented by each. The side shots that follow, represent an existing corner or record data based on their existing relationship to the highlighted control point.
Using this method, each traverse can be used to quickly populate the Double Proportion dialog, eliminating the need to enter any data manually.
As a rule, both traverses will show True Bearing and Ground Distance, but we mixed it up here to show that it doesn't matter how the traverses are formatted.
[[ Traverse View - Retrace 200500 (True Bearing, Ground Dist,
Feet) ]]
Ground Dist (at project elevation 1000.00)
True Bearing (mean course convergence PLSS)
Point Type True Bearing Ground Dist
Northing Easting
2200500M
349563.894 8427688.259
1000M SS N0°37'03"E 2671.86
352235.466 8427688.399
1001M SS S0°05'43"W 5343.96
344220.720 8427736.672
1004M SS S89°48'57"W 5289.99
349490.816 8422399.347
1007M SS S88°34'07"E 2661.68
349526.120 8430349.383
[[ Traverse View - Record 200500 (Grid Bearing, Ground Dist, Feet)
]]
Ground Dist (at project elevation 1000.00)
Grid Bearing (inverse grid coordinates)
PoinPoint Type Grid Bearing Ground Dist Northing
Easting
1001M 344220.720 8427736.672
200200500R N0°00'00"E
5280.00 349500.156 8427736.672
1001R SS S0°00'00"E 5280.00
344220.720 8427736.672
1004R SS S87°47'00"W 5245.68
349297.283 8422495.483
1000R SS N0°00'00"E 2640.00
352139.869 8427736.672
11007R SS S89°54'00"E 2639.34
349495.550 8430375.723
Retrace Data
Highlight point 200500M in the first traverse and choose COGO, Double Proportion. TPC looks for the words 'Retrace', 'Temporary' or 'Measured' in the traverse name. Point 200500M then becomes the Temporary Corner and the first four side shots that follow it become the existing corners, using the orientation of the side shots from 200500M to deteremine which side shot represents which existing corner in the dialog.
Record Data
Highlight point 200500R in the second traverse and choose COGO, Double Proportion. TPC looks for the word 'Recrord' in the traverse name. Point 200500R then becomes the Lost Corner in the dialog. The first four side shots that follow it are inversed to generate the record data. TPC uses the orientation of the side shots to deteremine which side shot represents which record data.
Related Topics
Single Proportion Dialog
Cadastral Survey
Professional