The 2017 Digital Terrain Model (DTM) is a 2 foot pixel
resolution raster in Erdas IMG format. This was created using the ground (class
= 2) lidar points and incorporating the breaklines.
The DTMs were developed using LiDAR data. LiDAR is an
acronym for LIght Detection And Ranging. Light detection and ranging is the
science of using a laser to measure distances to specific points. A specially
equipped airplane with positioning tools and LiDAR technology was used to
measure the distance to the surface of the earth to determine ground elevation.
The classified points were developed using data collected in April to May 2017.
The LiDAR points, specialized software, and technology provide the ability to
create a high precision three-dimensional digital elevation and/or terrain
models (DEM/DTM). The use of LiDAR significantly reduces the cost for developing
this information.
The DTMs are intended to correspond to the orthometric
heights of the bare surface of the county (no buildings or vegetation cover).
DTM data is used by county agencies to study drainage issues such as flooding
and erosion; contour generation; slope and aspect; and hill shade images. This
dataset was compiled to meet the American Society for Photogrammetry and Remote
Sensing (ASPRS) Accuracy Standards for Large-Scale Maps, CLASS 1 map accuracy.
The U.S. Army Corps of Engineers Engineering and Design
Manual for Photogrammetric Production (https://www.publications.usace.army.mil/Portals/76/Publications/EngineerManuals/EM_1110-1-1000.pdf)
recommends that data intended for this usage scale be used for any of the
following purposes: route location, preliminary alignment and design,
preliminary project planning, hydraulic sections, rough earthwork estimates, or
high-gradient terrain / low unit cost earthwork excavation estimates. The
manual does not recommend that these data be used for final design, excavation
and grading plans, earthwork computations for bid estimates or contract
measurement and payment.
This dataset does not take the place of an on-site survey
for design, construction or regulatory purposes.