Lab 4 Flashcards
What’s the difference between pulses and returns?
One emitted laser pulse can return to the lidar sensor as one or many returns. Any
emitted laser pulse that encounters multiple reflection surfaces as it travels toward the
ground is split into as many returns as there are reflective surfaces.
In terms of electromagnetic spectrum, what is different about bathymetric lidar lasers as
compared with airborne lidar lasers?
With a bathymetric lidar survey, the infrared light (traditional laser system) is reflected
back to the aircraft from the land and water surface, while the additional green laser
travels through the water column.
What formats of lidar data are supported in ArcGIS?
ArcGIS supports lidar data that is provided in either ASCII or LAS file format.
What affects intensity values?
It is based, in part, on the reflectivity of the object struck by the laser pulse. Reflectivity is
a function of the wavelength used, which is most commonly in the near infrared. The
strength of the returns varies with the composition of the surface object reflecting the return.
Intensity is relative, not quantifiable, therefore you cannot expect the same value
off the same target from flight to flight or from elevation to elevation.
What are the differences between LAS 1.0 and LAS 1.1-1.4?
Only LAS 1.1 and later has a predefined classification system in place. Unfortunately, the LAS 1.0 specification does not have a predefined classification scheme, nor do the files summarize what, if any, class codes are used by the points. You need to obtain this information from the data provider.
What are the three types of datasets that support lidar in ArcGIS, and what are the
storage locations, relationships to LAS source files, and lidar inputs for each of them?
a. LAS dataset
i. Storage locations: File system
ii. Relationship to LAS source files: Points to
iii. Lidar inputs: LAS files
b. Mosaic dataset
i. Storage locations: Geodatabase
ii. Relationship to LAS source files: Points to
iii. Lidar inputs: LAS files, terrain datasets, LAS datasets
c. Terrain dataset
i. Storage locations: Geodatabase
ii. Relationship to LAS source files: None
iii. Lidar inputs: Multipoint feature class
Which extension provides the most functionality for use of lidar in ArcGIS?
ArcGIS 3D Analyst extension
What is the difference between point spacing and point density?
point spacing is not the same as point density.
Point spacing (PS) is defined as linear units per point, and
point density (PD) is defined as points per square unit area.
To convert point spacing to point density use the following equation: PD = 1/(PS)^2 . A
higher point density means lower values for point spacing.
Describe why LAS datasets are an efficient method of viewing and accessing lidar.
- The LAS dataset provides immediate access to lidar data without the need for
data conversion or importing. - The LAS dataset is similar to a terrain dataset with on-the-fly surface views, but it
can also be used for point viewing and does not require preprocessing. You are
not required to do any processing to quickly understand the lidar data that you
are working with. - It is displayed as either a surface or a point cloud, which allows you to quickly
assess and understand the lidar data. - You are able to expose any data blunders or issues quickly due to the fast
display and statistical analysis. - The LAS dataset is scalable, allowing you to work with large point clouds quickly
and efficiently. The LAS dataset representation updates itself automatically as
you pan and zoom around the display.
What types of analysis can you perform with LAS datasets?
A. Analyze lidar points against first-return, bare earth, or any desired filter type.
b. Manipulate lidar point classification code values to fix classification errors.
c. Analyze lidar data using classification codes.
d. Run floodplain models against bare-earth lidar points.
e. Conduct statistical analysis on lidar points.
f. Export statistical information for further analysis.
g. Create raster DEMs and DSMs.
h. Create intensity images.
i. Estimate forest canopy density and height.
j. Run initial QA/QC processes on newly acquired postprocessed lidar data.
k. Measure heights between points.
l. Analyze lidar data as points or as a surface in 2D or 3D.
In ArcMap, what affects the amount of points you see in a LAS dataset?
When a LAS dataset is added to ArcMap, the point count is often too high to display
initially. Therefore, the minimum bounding boxes representing the extents of each LAS
file in the LAS dataset are shown. When a LAS dataset is going to be drawn the total number of points within the current extent is estimated. The number of points directly
corresponds to the amount of data on disk that needs to be scanned. The display will
switch to the wireframe view if the estimated number of points is too large and would
therefore take too long to read from disk.
