2012 FEMA Lidar: Southern Virginia Counties

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Survey report by the contractor.

This graphic shows the coverage of the 2011 USGS/FEMA Virginia Southern Counties lidar collection.

Date that the source FGDC record was last modified.

Converted from FGDC Content Standards for Digital Geospatial Metadata (version FGDC-STD-001-1998) using 'fgdc_to_inport_xml.pl' script. Contact Tyler Christensen (NOS) for details.

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Establishment of survey points to support the LiDAR data collection. two existing published NGS stations (FX5374, FX5377) were observed in a GPS control network and used to establish one new point for the primary control for this site. FX5374, FX5377, 239DEB05 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 South, 239DEB05, 36 50 47.15799, -76 59 00.22918, -14.6087, 20.3903 Counties South, FX5374, 36 48 32.84206, -76 37 15.00850, -28.4893, 7.6452 Counties South, FX5377, 36 41 06.67117, -77 06 58.40345, -7.2248, 27.6380

Airborne acquisition of Lidar GeoDigital 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 300.00 m Overlap 55 percent 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 Cessna 310 aircraft was used to conduct the aerial survey. The Cessna 310 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: 18 List Of Missions: o112005a o112006a o112006b o112007a o112007b o112008a o112008b o112010a o112010b o112012a o112013a o112014a o112014b o112015a o112015b o112016a o112016b o112018a Number Of Flight Lines: 219

- 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)

The NOAA Office for Coastal Management (OCM) received the topographic lidar files in LAZ format. 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 (As Needed).