FLOOD10473 - Laserfiche WebLink

Home Browse Search

, OCT-31-2003 08:41 ICON Eng ineer ing, Inc. 303 221 4019 P.02/06 CRYSTAl" RIVER DATUM ISSUE ICON received a fax [Tom Contract Surveyors that stated they used R-156 as a control point, along with a HARN Point located "about 10 miles away". A copy of the fax is enclosed. The tax included the latirude, longitude and elevation of point R-156 that Contract Surveyors IlJsed to control the mapping for the Crystal River. We downloaded the NGS Data Sheet for Point R 156 to get the physical description of the point. This Data Sheet is attached. We noticed that th~ published elevation differed from the elevation provided by Contract Surveyors. .----""" ".~. Contract Point R 156 Location Published NGS Data Sheet As Used By Surve"ors -" -- n Elevation 7447.29 NAVD 88 7444.488 ... N 39 07 S6 N 39 07 56.08800 La.llLong W 107 1545 W 107 1545.335443 ....... .....-- Description Standard Disk Stamped R 156 1934 in top of 4-ft Sq Boulder, ." ,..-. l-ft above ground - ,...... .. .., Bob Hennessey went to the field and located Point R 156. He set up ollr GPS Base Station using the el~vation provided by Contract Surveyors, He then went to the Crystal River area and conducted a map check, He was able to confirm the spot elevations shown on the mapping using the Contact Surveyor~ elevation for R 156, and also confirmed that the contours shown on the map seemed to accurately represent the general lay of tile land, Our concern is that. the published elevation for R 156 is reported to bc NA VD 88 and is approximately 2,8 feet higher than the elevation used by Contract Survcyors, Consequently, we belicve that the mapping as prepared by Kuchera is not on NA VD 88. As a point of information, the Data Sheet for R 156 confirms that the point was originally established as an NAVD 29 monument having an elevation of 7441.87. Thus it appears that the datum lIsed by Contract Surveyors is somewhere between the NGYD 29 and NA YD 88. ICON then d~termined the approximate latirude and longitude of the downstream limit and upstream limit of the Crystal River study reach, along with a third point roughly mid way between, This information was entered into the COE program CorpsCON. From this program, we weae able to confirm the conversion between NA VD 88 and NGVD 29, Based on an analysis of the average difference between the two datums, and based upon a review of the FEMA regulations, we found that the avt:rage conversion for the extreme points is less than 0,25 feet: variation b~tween the determined points. According to FEMA standards, it would thertefore be permissible to apply one conversion equation to elevate a hypothetical map of the Crystal River study reach from NGVn 29 to NA YD 88. See anachcd analysis, Based upon the foregoing, we believe that the Kuchera mapping can be corrected to NA VD 88 by applying a single correction factor of +2.8 feet to all points. In keeping with FEMA requirements, the FIS must include a statement signed by a PLS stating that the mapping meets FEMA standards and is based upon NA VO 88. We are somewhat reluctant to acc:ept this responsibility as ICON did not originally prepare the mapping, and Kuchera was under contract to furnish the mapping in NA VO 88, Pl~ase advise us as to how you wish ICON to prom:ed, We can not start the hydraulic analysis until we can be assured that the datum issue of the mapping has been resolved.