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About LIDAR > Answers to LIDAR Questions
Many of the questions related to the LIDAR project are similar in nature.
Below is a compilation of answers to frequently asked questions (FAQ).
1.1 What do all of these acronyms stand for?
- ALACE: Airborne LIDAR Assessment of Coastal Erosion
- ATM: Airborne Topographic Mapper
- CRS: Coastal Remote Sensing
- GIS: geographic information system
- ITRF 94: International Terrestrial Reference Frame of 1994
- LDART: LIDAR Data Retrieval Tool
- LIDAR: Light Detection and Ranging
- NAD 83: North American Datum of 1983
- NASA: National Aeronautics and Space Administration
- NAVD 88: North American Vertical Datum of 1988
- NGVD 29: National Geodetic Vertical Datum of 1929
- NOAA: National Oceanic and Atmospheric Administration
- USGS: United States Geological Survey
- UTM: Universal Transverse Mercator
- WGS 84: World Geodetic System of 1984
1.2 When is the ALACE project going to fly in my
state/county/region?
-
Check the maps on the Topographic
Change Mapping home page to see if we have already collected
data in your area of interest. If we have not, and your area is
not coastal (near a significantly large body of water), then we
most likely will not. Although the topography of inland areas
is certainly interesting, it is not within our purview. If your
area is coastal, then address your question by e-mail to csc@csc.noaa.gov.
1.3 If your maps show that you collected data in area X, but those data are
not in LDART, when can I expect to be able to download them?
-
The team at NASA Wallops Flight Facility collects and performs
the initial processing of the LIDAR data. Initial processing
takes a minimum of six weeks. Project partners and
collaborators then have access to the data for a period of
approximately six months while Quality Assurance/Quality Control
procedures are being conducted. Data will generally be
available to the public approximately six months after
collection. For questions about a specific area or data set,
send an e-mail to
csc@csc.noaa.gov.
1.4 If I would like to have the ATM flown over
an area in which I have a project study, to whom would I speak about
this?
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2.1 How far inland do you fly?
-
It depends on the width of the beach in question. Generally the
ATM is flown over a beach in four passes: two over the
sand/water interface, and two over the sand/developed area
interface. These four passes produce a 300- to 500-meter wide
swath.
2.2 How far over water do you fly?
-
We don't fly very far over the water generally because we don't
penetrate the water surface. The ATM has, however, been used to
map waves. For more information on this type of data, contact
William Krabill
at NASA Wallops Flight Facility.
2.3 Does the ATM penetrate the water surface
(i.e., do you collect bathymetry)?
2.4 Is the ATM collecting raster data
(imagery)?
-
No. The ATM returns many millions of georeferenced "x,y,z"
points. It takes a significant amount of processing to make
imagery from these data. LDART can create raster data from the
points collected by the ATM.
2.5 What is the density of points on the
ground?
-
This is highly variable, but densities are typically 3 points
per 10 square meters. The variability is due to the greater
number of points along the edges of a pass and where two passes
overlap.
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3.1
What is the horizontal/vertical accuracy of the data you
collect?
-
The horizontal accuracy is 0.8 meters (for a flight altitude of
700 meters), which means that the actual point could fall
anywhere within a 0.8-meter radius from the calculated position.
The vertical accuracy is about +15 centimeters for
measurements collected over the beach.
3.2 Why would LIDAR elevation data that I
downloaded be different from ground survey elevations that I collected
myself?
-
There are many possible reasons for differences in values:
- There may be a units problem. Make sure that your data
and the LIDAR data are both in the same system vertically. If
you are unsure about the LIDAR data, consult the metadata.
- There may be a datum/projection problem. Understanding datums and
projections can be difficult, and care must be taken to
ensure that all data are in a consistent datum and
projection. The metadata e-mail you received from LDART
will tell you what your horizontal and vertical datums are
for the LIDAR data.
- It is possible that our data are incorrect. If you have
found a drastic error (greater than the accuracy stated
above) or consistent error, we would like to know about it.
Please send an e-mail to csc@csc.noaa.gov.
3.3 When LDART delivers a data file with State
Plane X and Y values carried out to six decimal places, are the LIDAR
data really accurate to a millionth of a meter/foot?
- No they are not. The horizontal and vertical accuracies are 0.8
meters and 15 centimeters, respectively. It is
difficult to program LDART to "know" what the precision of the
data is in various projections and selection of units. For now,
we decided to put in more digits than necessary for some
projections, in order to accommodate the precision needed for
others.
3.4 Where the elevation values are carried out
to three decimal places, are all of the digits
significant?
-
The instrument measures the elevations to the millimeter level,
so we carried those digits through all calculations. Again, the
vertical accuracy is 15 centimeters, so use care in
interpreting differences smaller than this.
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4.1 If I never received e-mail from LDART, or my data file had nothing in it,
or LDART didn't work, what can I do?
-
You may have typed your e-mail address incorrectly, or LDART may have a bug. Try your request
again, and if you still do not receive any messages, send an e-mail detailing your problem to
csc@csc.noaa.gov.
4.2 Why isn't there a tool that allows me to draw a box of any size, at any
location? I can't seem to get exactly the bounds of the area I want with the map interface in LDART.
- The technology and the time to build such a system are limited, but we have plans to implement
such a system in the future.
4.3 Why don't you have downloadable data
files that I can click on without having to select an area?
-
We will have some files of this type as we create CD-ROMs and
make specialized data products for each region, but we thought
it would be more useful to the larger community if we allowed
users to select whatever geographic area they desired.
4.4 What do I do if I create a data file with
LDART, but it is too large for me to download or work
with?
-
Go back to LDART and choose a smaller area, or subsample your
data using the "binning" option.
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5.1 To what horizontal/vertical datum are these
data referenced?
-
When the data are collected and stored they are referenced to
the WGS 84 ellipsoid and the ITRF 94 datum. (Learn more about
the concept of a vertical datum.)
LDART will allow the user to select the datums (both vertical
and horizontal) and the units of measure. Refer to the metadata
in the e-mail delivered by LDART for the datum used in a
particular data set.
5.2 How can I get LIDAR data referenced to a
particular datum and projection?
-
LDART can deliver data referenced to the following horizontal datums:
LDART can deliver data referenced to the following vertical datums:
- Ellipsoid
- NAVD 88
- NGVD 29
LDART can deliver data projected to:
- Geographic (no projection)
- Any UTM zone in the United States
- Any coastal State Plane projection
Use the choice boxes in the LDART applet to select these parameters.
5.3 Why are the elevation values I downloaded
much less than zero (-20 to -40 meters)?
-
The data you downloaded are most likely referenced to an ellipsoid
surface. The values you are seeing are about the expected range
for that vertical reference frame. To get values that are closer
to "0=mean sea level" try either the NAVD 88 or NGVD 29 datum in the
Vertical Datum selection box in the LDART applet. For help with
the concept of a vertical datum see the National Geodetic Survey
FAQ site or this
University of Colorado site.
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