What is recommended good practice for ensuring the accuracy of a TGBM's elevation?

Measuring relative to at least three other local benchmarks is essential for ensuring the accuracy of a TGBM's elevation. This practice allows for the identification and correction of any discrepancies that may exist between the benchmarks. By involving multiple reference points, surveyors can mitigate the risk of relying on a single datum which may be affected by local variations such as subsidence or errors in previous measurements. The use of at least three benchmarks enables the establishment of a more reliable network, allowing for cross-validation of elevations and improving overall accuracy. This triangulation process helps to confirm that measurements are consistent and precise, which is critical in hydrographic surveying where accuracy is paramount for mapping and navigation purposes.

The other practices mentioned, while they may contribute to data collection or understanding of local conditions, do not fundamentally address the elevation accuracy in the same effective manner. Measuring against only one other local benchmark could lead to significant errors if that benchmark is inaccurate. Recalibrating water levels and documenting historical flood levels are valuable for other aspects of hydrographic studies but do not directly ensure the precise elevation of a TGBM as effectively as the comparative method involving multiple benchmarks does.