Civil Engineering Reference
In-Depth Information
not bargain for a parcel 999.60 feet square. Nor does she bargain for less area.
When a surveyor is retained to resurvey a parcel of land created in 1925, he must
follow in the footsteps of the original surveyor. He does not have the authority to
change the size of the footsteps.
Some jurisdictions require certain surveys to be connected to state plane coor-
dinates. Many state statutes also provide that state plane coordinates are an accept-
able means of locating a boundary corner and that said coordinates constitute a
legal and satisfactory description of the corner. These statutes usually contain a
provision that no purchaser or mortgagee of real property is required to depend
solely on state plane coordinates to describe a corner. A purchaser may still require
physical monuments—and they should. When ground values are shown on a plan
and the survey is tied into a state plane coordinate system, the surveyor must be
careful to show enough information so that anyone interpreting the plan will know
how to reproduce the survey on the ground and to connect it to state plane. This
must include clear descriptions and identifications of control points, basis of the
coordinate system and units, scaling factors and ties from the control to the parcel
being described.
12.2 Low Distortion Projections
Low Distortion Projections (LDP) are becoming a common alternative to state
plane coordinates, particularly in mountainous areas having high elevations which
cause substantial distortion. An LDP is simply a coordinate grid which is located
at a height above the ellipsoid which is the approximate average or median eleva-
tion of the topography where the survey will take place.
A local central meridian is selected at the representative ground surface height
as shown in Fig.
12.7
. To minimize distortion, the coverage of the LDP is designed
to only cover the area of interest. The low distortion projection plane shown in
Fig.
12.7
is midway between the lowest and highest elevation (which may not be
the average elevation). The custom scaling factor would be valid for the darkly
hatched area. The LDP allows a single scaling factor to be used for the entire pro-
ject. The LDP has the advantage of reducing the differences in distances meas-
ured with GPS and total stations. This feature is a great advantage over state plane
coordinate systems, particularly when working in areas with high elevations.
Some surveyors believe that with the high accuracy possible with GNSS, and the
issues between ground measurements versus state plane, the state plane systems
are becoming obsolete and LDP may offer a better solution. Many data collectors
and surveying calculation programs now allow a user to create custom LDPs with
conversions to state plane coordinates. A number of states are considering offering
multiple LDPs for their states. Until LDPs become a part of the legal framework
of a jurisdiction, there is no official basis for a particular LDP. In order to keep
distortion to a minimum the LDP area must be smaller than a state plane system so
many LDPs will be required for an area the size of a state.
