Remote Sensing: LIDAR: Acquisition and Processing Overview

Acquisition and Processing

Because every laser/digital imagery acquisition project has different requirements, each one is carefully planned to meet the customer’s exact specifications. All projects go through the same requirements of planning, execution and quality control standards to ensure the successful data capture and processing. Following is a brief description of the closely monitored steps taken by Spectrum to ensure the success of each and every project flown.


Mission Planning

Mission planning consists of several steps that ensure proper flight preparation. These include information about: elevation data, vegetation coverage and cultural features; FOV (field of view) and GSD (ground sample distance) requirements; and PDOP (positional dilution of precision or GPS quality).


Calibration Site

For all projects we set up a calibration site, which is flown before each LIDAR mission. The calibration site consists of an area with existing control placed by static GPS methods. Points are photo identifiable so they can be located in the digital imagery as well.


Airborne GPS Reference Stations

Prior to flight and data acquisition, all airborne GPS reference stations identified for use in controlling the project area will be submitted to the client for review and approval. Base stations are set within a radius that guarantees the accuracies required for a particular project are met.


Data Acquisition

The flight crew is guided by a GPS controlled flight management system, which displays the flight plan, altitude, heading, crosstrack deviation and PDOP. During flight, the crew monitors all functions in system operation to ensure a successful mission.


Field Processing/ Coverage Verification

At the end of the flight day all data is copied to a second set of data drives or tapes for archival purposes. Two copies of all data are maintained throughout our entire process. Calibration flight information acquired for the LIDAR and digital imagery is also verified in the field to ensure all equipment is properly functioning. Coverage verification is accomplished using the post-flight processed GPS solution. These drives are forwarded to Spectrum’s offices for validation and QA/QC before processing begins. Both horizontal and vertical aircraft position and orientation are verified and plotted against the 3-D flight plan to ensure proper coverage of topographic and image data. This initial quality control is significant. It ensures that all of the data acquired is correct and complete. If not, specific areas may be re-flown while the flight crew is still on site, eliminating costly re-capture procedures with traditional film developing techniques.


Office Processing

Spectrum has developed proprietary GIS software used to process the laser points through to final output. Because of the vast amounts of laser data collected (5-foot GSD yields about 1,115,000 points per square mile) various quality control steps are built into our processing procedures to assure there are no systematic errors.


Image Processing

The GPS/IMU information acquired for the laser data is used to position the digital images. All images are time tagged to within a few milliseconds of their true time, which equates to only a few centimeters on the ground. The GPS position and the rotation angles from the IMU determine the position and attitude of each frame to photogrammetric quality. The images are QA/QC’d through photogrammetric software where the calibration site control is checked in stereo to verify processing.

Image showing digital ortho draped over DEM.

Bare Earth Determination

Bare earth filtering is required in order to generate contours. A laser processing professional will decide which classification technique(s) to apply for each discrete area type, to best portray a bare earth surface. Factors that affect this decision are slope, vegetation and cultural features. Each project has unique characteristics that can only be assessed after the data is collected. After visualizing results from the selected filtering techniques, using GIS techniques like shaded reliefs, 3-D viewers and elevation images, results are reviewed in stereo using softcopy photogrammetric software. When it is determined that the filtering process is yielding the best possible bare earth surface for each of the area types, filtering of all data commences.

Before LIDAR point classification

After LIDAR point classification

Because of the massive amount of point data acquired and processed, Spectrum’s classification methods results in a more accurate bare earth ground terrain model than traditional photogrammetric methods. Traditional softcopy accuracy is limited by the number of points an operator is able to accurately place.


info@specmap.com