Forensic Audit Report-WPCA Phase IV, Part B. Contract ... - Trumbull

Forensic Consulting Services Report
Town of Trumbull, Connecticut
Page 14
It is important to note that “Ortho-photographic” type mapping is generated by utilizing aerial
photography taken at various flight altitudes based on the desired needs and scale of the mapping
applications being produced. These photos are then planimetrically corrected (or “orthorectified”) to
produce a more accurate representation of the earths’ surface, having been adjusted for topographic
relief, lens distortion, and camera tilt. A digitization process then transforms the corrected data into
a usable electronic format for actual design, such as AutoCAD.
One of the more important factors controlling the mapping accuracy of the end product is the flight
altitude at which the original aerial photos are taken. Typically, aerial photos taken at higher
altitudes produce less than desirable results, especially in relation to accurately defining topographic
and physical features. Due to its limitations for accuracy and attention to detail, the higher altitude
“Ortho-photographic” type of mapping is generally used and deemed more appropriate for planning
purposes (100-scale and greater) and is not typically used for detailed design applications where the
need for accuracy and attention to detail is more of a controlling element of the ultimate design.
Also, the age of the aerial photos and the specific production techniques used to produce the
digitized mapping become a factor mostly due to the overall advancements in the technology in
more recent years. Simply put, securing and utilizing current aerial photography complied with the
most current and advanced technology of today’s industry produces a much better and more accurate
end product than that which may have existed ten to twenty years ago.
In the case of Contract 3, we believe the generalized sewer design effort noted above appears to be a
direct result of less than adequate and possibly outdated base mapping which was produced and used
by the design engineer during the design process. As stated in the original 1996 Agreement, whether
currently considered lacking or not, “these plans” were “the basis of design”. Although the design
mapping used is in the specified 1-inch equal 40-foot horizontal scale, which is relatively common
for utility based designs, the accuracy and attention to planimetric details is somewhat lacking (i.e.
physical features, structures, drainage facilities, watercourses, utilities, vegetation, limits of
pavement, etc.).
To this extent, although the original 1996 Engineering Agreement clearly states items such as 2-foot
contours, drainage facilities with invert elevations (i.e. manholes, catch basins, culverts, etc.),
random spot elevations, existing utilities, limits of significant vegetation were to be provided as a
supplement of the overall design, these attributes are limited, and in some cases nonexistent on the
design drawings.
Specifics of our investigations reveal the following:
‣ Contours, which appear to have been required for all areas of the design contract, roadways
and cross country routings alike, were only provided for cross country routings. Where
contours are shown, they are not labeled in regards to any specific elevation of reference
thereby leaving one to cross reference approximate elevations with the graphical elevations
delineated on the design profiles.