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---- 3.1.3 Import As-Built Topography

Last Updated: 06 December 2023         For: OptiSurface Designer V3.5+

Overview: The As-Built feature allows comparison of the Proposed Topography and the post operation topographic survey to check if the design is within acceptable tolerance, however, you need to be careful because we are pushing the limits of GPS accuracy when we are doing landforming. The real test is after you have put furrows in the field and get irrigation or rainfall/runoff event and the furrows should drain and no standing water.

Having said that, you can do the As-Built Check acknowledging the accuracy limits of the RTK GPS which should be around +/- 20mm if you have the RTK base station at the field edge for survey and landforming. The accuracy if the GPS over 15mins is probably closer to +/-10mm. For this reason, there should be one or two passes down the field in the direction of the furrows (uphill and downhill) to get the best result.

Displayed By: Click File Menu > Import As-Built Topography...


As-Built Topography - the post operation topographic survey when imported as-built will show up here.
IMPORTANT: It should have identical master benchmark to the field being compared to.

As-Built Cut/Fill - generate the Cut/Fill map by by right clicking As-Built Cut/Fill in Project Explorer > Re-calculate... > use the same inputs as the landform design calculations for Main Slope Grid Bearing (deg), Calculation Grid Spacing (m or ft), and tick ON Rebalance > OK. This will adjust the As-Built Topography elevation up or down to closely match the Proposed Topography and subtract it to come up with the updated Cut/Fill Map.

The Proposed Topography and the As-Built Topography will be compared and earthworks will show in the Result Window. Ideally, the earthworks should be within 50 m3/ha and no single cut/fill should be greater than 50mm. HOWEVER, YOU CANNOT SUBTRACT THE EARTHWORKS VOLUME FROM WHAT YOU WANT TO PAY THE CONTRACTOR BECAUSE THIS IS THE LIMIT OF GPS ACCURACY.  

NOTE: The As-Built Topography and Cut/Fill Map Properties follows the Existing and Proposed Topography or Cut.Fill Map Properties. You will need to Re-import the post-op survey or Re-calculate the Cut/Fill Map so changes in the Properties take effect.

Another check that you can do is to set the contour interval of the As-Built Topography to 50mm (0.05m) and look for any contour lines that are circular and do not extend to the boundary. These can indicate depressions or ridges. You can also import the As-Built survey and run the drainage analysis with furrows on it to see if there are any depressions more than say 20mm.

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