2012 FEMA Lidar: Middle Counties (VA)

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This graphic shows the coverage of the 2012 FEMA Middle Counties (VA) lidar collection.

Date that the source FGDC record was last modified.

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Establishment of survey points to support the LiDAR data collection. 3 existing published NGS stations (GV1969, DL3187, 1110306) were observed in a GPS control network and used to establish 4 new points for the primary control for this site. GV1969, PTBAO,239DEB01, 239DEB02, 239DEB03 were observed and used to control all flight missions and static ground surveys. The following are the final coordinates of the control points used for this project: SurveyBlock, Station, Latitude(D M S Hem), Longitude(D M S Hem), H-Ell(m), H-MSL(m) Counties Middle, 239DEB01, 37 12 14.83506, -77 11 44.20093, 6.4368, 40.2481 Counties Middle, 239DEB02, 37 26 38.58484, -77 02 35.39947, -24.5572, 9.4443 Counties Middle, 239DEB03, 37 13 14.10984, -77 11 02.23058, 0.7420, 34.5500 Counties Middle, PTB-AO, 37 11 05.10614, -77 29 59.30881, 24.9968, 58.4983 Counties Middle, GV1969, 37 11 59.24638, -77 27 47.44373, 21.1918, 54.7664

Airborne acquisition of Lidar

Terrapoint used one Optech ALTM 3100EA system to collect the data.

The Optech System was configured in the following method:

Aircraft Speed 150 knots

Data Acquisition Height 950 m AGL

Swath Width 691.54 m

Distance between Flight Lines 311.2 m

Overlap 55 %

Scanner Field Of View 22 +/- degrees (+/-2 degrees flagged as withheld)

Pulse Repetition Rate 70 KHz

Scan Frequency 40 Hz

Number of Returns per Pulse 4 Discrete returns

Beam Divergence 0.3 mRad

Flight Line Length shorter than 70km

Base Station Distance shorter than 40km

Resultant Raw Point Density ~2 pt/m2 with overlap

Aircraft platforms were used in the collection of this project:

A Piper Navajo aircraft was used to conduct the aerial survey. The Navaho is a fixed wing aircraft that have an endurance of approximately 5 hours.

-GPS-IMU: High accuracy IMU (200Hz) and GPS information (1Hz) concerning the attitude and position of the sensor were acquired at the same time as the Laser data.

Ground based GPS stations also acquired consecutive GPS information for the duration of the flights.

A combination of Sokkia GSR 2600 and NovAtel DL-4+ dual-frequency GPS receivers were used to support the airborne operations of this survey.

- Flights and Flight Lines

The area is covered by several missions, a mission is defined as the block of acquisition between aircraft take-off and landing flown under good meteorological and GPS conditions, and each mission includes multiple flightlines.

Number Of Missions: 27

List Of Missions: o211126a o211127c o211130a o511127b o212090a o212091a o212092a o212902b o212093a o212093b o311344a o311344b o311345a 311345b o311347a o311347b o311348a o311348b o311349a o311351a o311351a o311352a o311352b o311353b o311354b o311362a o311363a o311363b

Number Of Flight Lines: 300

Airborne GPS Kinematic processing

Airborne GPS kinematic data was processed on-site using GrafNav kinematic On-The-Fly (OTF) software. Flights were flown with a minimum of 6 satellites in view (13o above the horizon) and with a PDOP of better than 4. Distances from base station to aircraft were kept to a maximum of 40 km, to ensure a strong OTF (On-The-Fly) solution. For all flights, the GPS data can be classified as excellent, with GPS residuals of 3cm average but no larger than 10 cm being recorded.

The Geoid09 geoid model, published by the NGS, was used to transform all ellipsoidal heights to orthometric.

Generation and Calibration of laser points

Laser data points are generated using Optechs software Dashmap. Those software combine the raw laser range and angle data file with the finalized GPS/IMU trajectory information.

Each mission is evaluated in Terrasolids Terramatch software to correct any residual roll pitch heading misalignments, if necessary those values are to the data.

The resulting point cloud is projected into the desired coordinate system and created in LAS format. One file per swath, files bigger than 2Gb split in 2.

On a project level, a coverage check is carried out to ensure no slivers are present.

Mission to mission adjustments of Lidar data

All missions are validated and adjusted against the adjoining

missions for relative vertical biases and collected GPS static and kinematic

ground truthing points for absolute vertical accuracy purposes.

Deliverable Product Generation

Raw Lidar point are projected were reprojected from UTM zone 18 to the delivery projection State Plane Virginia, US Survey Feet.

*Raw Calibrated LIDAR Point Cloud

Raw LiDAR point cloud, was provided in the following formats/parameters:

- LAS V1.2, point record format 1, Adjusted GPS time, georeferencing information populated in header

- The following fields are included in the LAS file:

1. Adjusted GPS time reported to the nearest microsecond

2. Flight line ID

3. Easting (reported to the nearest 0.01ft)

4. Northing (reported to the nearest 0.01ft)

5. Elevation (reported to the nearest 0.01ft)

6. intensity

7. Echo number

8. Classification

9. Scan angle

10. Edge of scan

11. Scan direction

- Full swaths, all collected points delivered (except discarded flightline)

- The Withheld bit flags the last 2 degrees of the swath (Additional areas are classified with the withheld in areas where wind or vegetation affected the quality of the data long the edge of the flight line. The classification of the additional withheld areas does not affect the density of the data.)

- 1 file per swath, 1 swath per file (except when swath had to be divided in section for size or calibration)

Dewberry utilizes a variety of software suites for inventory management, classification, and data processing. All LiDAR related processes begin by importing the data into the GeoCue task management software. GeoCue allows the data to retain its delivered tiling scheme (5000 ft by 5000 ft). The tiled data is then opened in Terrascan where Dewberry uses proprietary ground classification routines to remove any non-ground points and generate an accurate ground surface. The ground routine consists of three main parameters (building size, iteration angle, and iteration distance); by adjusting these parameters and running several iterations of this routine an initial ground surface is developed. The building size parameter sets a roaming window size. Each tile is loaded with neighboring points from adjacent tiles and the routine classifies the data section by section based on this roaming window size. The second most important parameter is the maximum terrain angle, which sets the highest allowed terrain angle within the model. Once the ground routine has been completed a manual quality control routine is done using hillshades, cross-sections, and profiles within the Terrasolid software suite. After this QC step, a peer review and supervisor manual inspection is completed on a percentage of the classified tiles based on the project size and variability of the terrain. After the ground classification corrections were completed, the dataset was processed through a water classification routine that utilizes breaklines compiled by Dewberry to automatically classify hydrographic features. The water classification routine selects ground points within the breakline polygons and automatically classifies them as class 9, water. During this water classification routine, points which are in close proximity (2 ft) to the hydrographic features are moved to class 10, an ignored ground. In addition to classes 1, 2, 8, 9, and 10, the project allows for a Class 7, noise points. This class was only used if needed when points could manually be identified as low/high points. Dewberry also used Class 11 - Withheld points.

The fully classified dataset is then processed through Dewberry's comprehensive quality control program.

The data was classified as follows:

Class 1 = Unclassified. This class includes vegetation, buildings, noise etc.

Class 2 = Ground

Class 7= Noise

Class 8= Model Key Points

Class 9 = Water

Class 10= Ignored Ground

Class 11= Withheld [later changed to class 15]

The LAS header information was verified to contain the following:

Class (Integer)

GPS Week Time (0.0001 seconds)

Easting (0.01 ft)

Northing (0.01 ft)

Elevation (0.01 ft)

Echo Number (Integer 1 to 4)

Echo (Integer 1 to 4)

Intensity (8 bit integer)

Flight Line (Integer)

Scan Angle (Integer degree)

Dewberry used GeoCue software to develop raster stereo models from the LiDAR intensity. The raster resolution was 1ft.

LiDAR intensity stereopairs were viewed in 3-D stereo using Socet Set for ArcGIS softcopy photogrammetric software. The breaklines are collected directly into an ArcGIS file geodatabase to ensure correct topology. The LiDARgrammetry was performed under the direct supervision of an ASPRS Certified Photogrammetrist. The breaklines were stereo-compiled in accordance with the Data Dictionary.

The data dictionary defines Tidal Waters as the land and water interface at the time of LiDAR acquisition of tidally influenced bodies of water. There is no minimum area requirement. Tidal variations over the course of a collection or between different collections will result in discontinuities along shorelines. This is considered normal and these anomalies should be retained. Variations in water surface elevation resulting in tidal variations during a collection should NOT be removed or adjusted, as this would require either the removal of valid, measured ground points or the introduction of unmeasured ground into the DEM. The USGS priority is on the ground surface, and accepts there may be occasional, unavoidable irregularities in water surface.

Breaklines must be captured at or just below the elevations of the immediately surrounding terain. Under no circumstances should a feature be elevated above the surrounding LiDAR points.

If it can be reasonably determined where the edge of water most probably falls, beneath the dock or pier, then the edge of water will be collected at the elevation of the water where it can be directly measured. If there is a clearly-indicated headwall or bulkhead adjacent to the dock or pier and it is evident that the waterline is most probably adjacent to the headwall or bulkhead, then the water line will follow the headwall or bulkhead at the elevation of the water where it can be directly measured. If there is no clear indication of the location of the waters edge beneath the dock or pier, then the edge of water will follow the outer edge of the dock or pier as it is adjacent to the water, at the measured elevation of the water.

Breaklines shall snap and merge seamlessly with linear hydrographic features.

The NOAA Office for Coastal Management (OCM) received the topographic lidar files in LAS format from USGS/FEMA. The files contained lidar easting, northing, elevation, intensity, return number, etc. The data was received in Virginia State Plane South 4502 (US ft) and NAVD88 (US ft). OCM performed the following processing for data storage and Digital Coast provisioning purposes:

1. The files were reviewed and erroneous elevations were removed.

2. Class 11 points (withheld) were reclassified to Class 15.