For DTM volumes it is absolutely critical that the raw data given to the computer is as perfect as possible. Gross inconsistencies will be automatically rejected in the program but, because of the infinite variations that are possible in topography, it can only reject huge anomalies. An incorrect tache shot will produce a localised hole or error in the contours. This may not be disastrous and may in fact not even be noticeable, but the same mistake will render an entire volume calculation completely useless.
Always check your volume result. On a straight forward site of a reasonable size, the quantity will generally be within 1% of the correct volume. This standard is reproducible on any other site providing you use common sense and maintain a clear idea of what is being measured.
The recommended method of checking DTM volumes is to use cross-sections. Errors ,(uch as those described in the first paragraph, will usually be readily identifiable during a visual inspection of the sections. Another DTM calculation of a different type will in all probability simply reproduce the error.
There are also several potential pitfalls to be aware of when calculating DTM volumes. These are :
The computer interpolates across a 'cutback' in the perimeter, thereby artificially increasing the surveyed area and therefore the volume. The reverse case is also possible. To overcome this situation, if the perimeter points are defined by comment, extract them and/or rerun the volume with a shorter search distance. Remember that this solution will affect the Datum volume but have very little effect on the Perimeter volume.
A corollary of this circumstance is a twin dump situation. In this case, use either a graphical clip or name filter facility to isolate one dump at a time. Both Perimeter and Datum volumes will be affected unless the perimeters lie on an even plane surface. In this case, the Perimeter volume would be unaffected.
A site with an imbalanced shape can be subject to error. Remember that the grid is derived from a vertical scan of the data. Therefore, a long North/South edge will result in a decrease in the calculated volume. Depending on the size of the site this can be negligible or substantial. To minimize this symptom, decrease the grid spacing or rotate the data through 90 degrees with a coordinate transformation or swing.
Grid interpolations are based on scanning the break line data in a North/South direction. This means that lines running in an East/West direction have more effect on the grid than North/South lines. Theoretically, a North/South terrain line could even fall entirely between two grid lines and not be used at all. Therefore, depending on the nature of the topography, consideration should be given to rotating a site through an angle in order to obtain more representative line crossings. This would have particular relevance where a site may have significant terraces or design lines oriented in an exact North/South direction.
Always be very careful of a Perimeter volume. It yields a quick easy answer but is subject to several limitations and is therefore inherently unreliable if not used in controlled circumstances.
Datum volumes are generally reliable with the proviso that the perimeter definition is critical. This is particularly so where repetitive measurements are made on the same area. A very small difference in the outside figure will yield a very big difference in a comparative volume.
Where a site has to be measured repetitively, it is best to establish a file of 'Library' perimeter points within which the measured site will always fall. This is then spliced into all future measurements and ensures that the defined perimeter is, not just similar by appearance, but also identical to the computer.