Data Set ID: 
SNEX17_TLS_PC_CRREL

SnowEx17 CRREL Terrestrial Laser Scanner (TLS) Point Cloud, Version 1

This data set contains terrestrial LIDAR survey (TLS) point cloud data collected at Grand Mesa, Colorado as part of the 2017 SnowEx campaign. Data were collected in the fall (September and October) and winter (February) seasons. Each point contains X, Y, and Z coordinates (Easting, Northing, and Elevation), along with ancillary information, such as intensity (i) and color (R,G,B), where available. This is unprocessed data which has not been classified by land use (e.g. bare earth, low vegetation, trees).

This is the most recent version of these data.

Version Summary: 

new data set

STANDARD Level of Service

Data: Data integrity and usability verified

Documentation: Key metadata and user guide available

User Support: Assistance with data access and usage; guidance on use of data in tools

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Parameter(s):
  • LIDAR > Lidar
Data Format(s):
  • Binary
Spatial Coverage:
N: 39.05, 
S: 39.012, 
E: -107.93, 
W: -108.22
Platform(s):GROUND-BASED OBSERVATIONS
Spatial Resolution:
  • Varies x Varies
Sensor(s):TLS
Temporal Coverage:
  • 26 September 2016 to 17 February 2017
Version(s):V1
Temporal ResolutionNot applicableMetadata XML:View Metadata Record
Data Contributor(s):Hiemstra, C. and A. B. Gelvin.

Geographic Coverage

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As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Hiemstra, C. and A. B. Gelvin. 2019. SnowEx17 CRREL Terrestrial Laser Scanner (TLS) Point Cloud, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/YOIPYEWCZOD5. [Date Accessed].
Created: 
24 July 2018
Last modified: 
27 February 2019

Data Description

Parameters

This data set contains unprocessed lidar point cloud data. Available parameters are shown in Table 1. 

Table 1. Parameters
Parameter
Description
X
X coordinate / Easting (meters)
Y
Y coordinate / Northing (meters)
Z
Elevation above sea level (meters)
Intensity
Pulse return magnitude
ReturnNumber
Integer number of the point in a sequence of multiple returns from a given outgoing pulse, values range from 1 to n
1 = first return
= NumberOfReturns
NumberOfReturns
Total number of returns from a given outgoing pulse
ScanDirectionFlag
Direction of motion during the outgoing pulse:
0 = right to left
1 = left to right
EdgeofFlightLine
Flag denoting that this point is at the edge of the flight line
Classification
Point class attribute:
0/1 = unclassified
2 = ground
3 = low vegetation
4 = medium vegetation
5 = high vegetation
9 = water
ScanAngleRank
Angle (-90° to +90°) at which the outgoing pulse was emitted from the TLS (0 = nadir)
UserData
N/A for this data set
PointSourceID
N/A for this data set
Red* Percentage of red color in the image, scaled 0 to 255
Blue*
Percentage of blue color in the image, scaled 0 to 255
Green*
Percentage of green color in the image, scaled 0 to 255
*Red, Blue, and Green (R, G, B) color data derived from coincident color photographs. Under snow-on conditions, light conditions were not suitable for color photographs so no color data is available. 

File Information

Format

TLS data are provided in Lidar Data Exchange zipped (.laz) files. 

Extensible Markup Language (.xml) files with associated metadata are also provided.

File Contents

The LAZ files consist of thousands of individual points, each of which contains the parameters and properties described in Table 1. Figure 1 contains a sample of these point values, as presented in a text file.

Figure 1. Sample of SNEX17_TLS_PC_CRREL_09262016_SiteI.laz contents presented in a text file.

Naming Convention

The data files are named according to the following convention and as described to Table 2:
SNEX17_TLS_PC_CRREL_[MMDDYYYY]_[location].laz

Example:
SNEX17_TLS_PC_CRREL_10022016_SiteB.las

Where:

Table 2. Naming Convention
Variable Description
SNEX17_TLS_PC_CRREL Short name for SnowEx17 CRREL Terrestrial Laser Scanner 
MMDDYYYY Date in MMDDYYYY format
location Location of scan

File Size

The .laz files range from approximatley 52 to 644 MB. The total file volume is approximately 4 GB.

Spatial Information

Coverage

Northernmost Latitude: 39.05° N
Southernmost Latitude: 39.00° N
Easternmost Longitude: 107.93° W
Westernmost Longitude: 108.22° W

Resolution

Spatial resolution is 4 mm at close range, but expands to 1-10 cm at distances greater than 100 m from the scanner.

Geolocation (XY Coordinates)

All data has been projected into the WGS 84/UTM Zone 13 North coordinate system, a detailed description of which is provided in Table 3.

Table 3. Geolocation Details
Geographic coordinate system WGS 84
Projected coordinate system WGS 84/UTM Zone 13N
Longitude of true origin -105
Latitude of true origin 0
Scale factor at longitude of true origin 0.9996
Datum WGS 1984
Ellipsoid/spheroid WGS 84
Units meter
False easting 500000
False northing 0
EPSG code 32613
PROJ4 string +proj=utm +zone=13 +datum=WGS84 +units=m +no_defs
Reference https://epsg.io/32613

Vertical Projection (Z Coordinates)

The elevation (Z coordinate) of each point is reported as the orthometric height, calculated based on the NAVD88 vertical datum and GEOID12B model. This value is different than the geodetic height associated with the WGS 84/UTM Zone 13N projection described in Table 3.

Temporal Information

Coverage

26 September 2016 to 17 February 2017

Resolution

Scans were completed once in the fall (September and October) and once in the winter (February).

Data Acquisition and Processing

Background

A terrestrial lidar survey (conducted with a terrestrial laser scanner) was performed at a number of sites around Grand Mesa, Colorado. The goal was to cover as much ground as possible, to scan into the trees as much as possible, and to produce a high-resolution bare-earth and snow-surface data set. To this end, surveys were conducted under both snow-off (September and October) and snow-on (February) conditions at both open and forested sites. Table 4 contains more information about survey locations and dates.

Table 4. Survey Locations and Dates
Snow-Off Conditions
(Fall 2016)
Snow-On Conditions
(Winter 2017)
Location Survey Dates Location Survey Dates
Site A 28 Sept. 2016 Site A 09 Feb. 2017 and 10 Feb. 2017
Site B 02 Oct. 2016 Site B 16 Feb. 2017
Site I 26 Sept. 2016 Site D 08 Feb. 2017 and 16 Feb. 2017
Site M 29 Sept. 2016 and 01 Oct. 2016 Site M 14 Feb. 2017
Site N 27 Sept. 2016 Site N 15 Feb. 2017
Ranger Station 01 Oct. 2016 Ranger Station 17 Feb. 2017
Local Scale Observation Site (LSOS) 02 Oct. 2016 Local Scale Observation Site (LSOS) 13 Feb. 2017
County Line Parking Lot 17 Feb. 2017

Acquisition

At each survey location, 4-15 scans were conducted using a Leica ScanStation C10 terrestrial laser scanner (TLS). The TLS was positioned to maximize the overlap between scans at the same site. Once combined, the overlapped scans create an area of continuous coverage. 

Each scan utilized a 360° horizontal- and a 270° vertical-arc sweep, collecting points from multiple surfaces and, when light conditions permitted, collecting (RGB) color images of the scanned area. Scans were completed at an intermediate resolution, meaning that at 100 m from the scanner, there was a 10 cm gap between adjacent points.

During each scan, 4-6 Leica six-inch High Definition Surveying (HDS) targets were distributed throughout the scan area. These targets serve as tie points for georeferencing the scans to real-world coordinates. Whenever HDS targets became hidden from scanner view, they were renamed and redeployed in a leapfrog fashion. A minimum of four reflectors were visible for each scan.

In addition to Leica ScanStation C10 TLS, a Trimble R8 GNSS base station was used at each site. The base station ran for the duration of each survey (for a minimum of two hours), collecting internal GPS data to use in post-processing. A Trimble R8 rover was also deployed to conduct a real-time kinematic (RTK) survey of the HDS targets. The base station broadcast correction data to the rovers, allowing for real-time corrections of atmospheric conditions and multi-path errors.

Processing

RTK survey data processing steps:

  1. RTK survey data underwent post-processing in Trimble Business Center.
  2. The internal file from the base station was sent to the Online Position User Service (OPUS), which is operated by the National Oceanic and Atmospheric Administration (NOAA) National Geodetic Survey (NGS).
  3. OPUS automatically corrected the data from the Trimble R8 rovers with the internal GPS file collected by the base station. Corrected GPS coordinates were accurate to within a few centimeters.

TLS data processing steps:

  1. A new database was created in Leica's Cyclone software.
  2. TLS data files, including point cloud data, images, HDS target scans, and any data associated with scanner placement, were imported into Leica's Cyclone software directly from the TLS. The aggregate files from each scan were stored in a folder called ScanWorld.
  3. Within each ScanWorld, a scanner-centric coordinate system was established.
  4. During registration, multiple ScanWorlds were integrated into a single coordinate system using overlapping HDS targets, resulting in a project-centric coordinate system.
  5. The Registration Mean Absolute Error (RMAE) was calculated for each constraint in the project-centric coordinate system.
  6. If RMAE was too high (>11 mm), the registration process was repeated to generate a new project-centric coordinate system, after which the RMAE was recalculated. This step was repeated until an acceptably low RMAE (typically between 2 and 11 mm) was obtained.
  7. The registration process was repeated once more using the corrected RTK survey data for the HDS targets to convert the point cloud data into a real-world global coordinate system. 
  8. Minor edits were made to the point cloud data, including removing extraneous points caused by scanning the sun, the top handle on the scanner, or objects in mid-air.
  9. The point cloud data was exported from Leica's Cyclone into one of three file formats, .pts, .ptx, or .txt.
  10. PointZip, a third-party software, was then used to export the point cloud data into the Lidar Data Exchange (.las) file format.
  11. Data were compressed into Lidar Data Exchange zipper (.laz) files.

Quality, Errors, and Limitations

The error range for this data set is 6 to 15 mm.

Instrumentation

Description

Surveys were conducted using a Leica ScanStation C10 terrestrial laser scanner. Under clear line-of-sight conditions, the C10 TLS has a range of approximately 100 m. For more information about this instrument, please see the Leica Geosystems website.

A Trimble DGPS R8 rover and base unit were used to collect position data for each HDS target. More information about these instruments can be found on the Trimble Products and Solutions website.

Software and Tools

TLS data were processed using Leica's Cyclone software. Cyclone is a composite of different software that can be mixed and matched to meet user needs. The two basic modules are Cyclone-REGISTER, which aligns point clouds captured from different scanning positions, and Cyclone-Survey, which provides measurement tools to analyze laser scan data.

PointZip was used to export TLS data to the Lidar Data Exchange file format.

Trimble Business Center software was used to correct the RTK survey data.

Related Data Sets

Other SnowEx Data Sets

Related Websites

NASA SnowEx

Contacts and Acknowledgments

Christopher Hiemstra
Cold Regions Research and Engineering Laboratory
Alaska Projects Office
PO Box 35170

Fort Wainwright, AK 99730-0170

Arthur Gelvin
Cold Regions Research and Engineering Laboratory
Alaska Projects Office
PO Box 35170

Fort Wainwright, AK 99730-0170

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