Airborne videography is a cost-effective and fast remote sensing technique for gathering information. Collected data can be easily input to a GIS for use in a variety of applications. Airborne videography systems are typically used for monitoring/mapping out linear features, e.g., roads, pipeline routes, power lines, river networks, coastlines, etc. in the visible part of the spectrum. However, these systems can now be extended to map out objects which are areal/regional in extent, e.g., forest stands, land parcels in the infrared and thermal parts of the spectrum.
      For any videography to be meaningful, the geographical location of the scene must be identified and, preferably, inextricably and unambiguously linked to the video signal. GPS provides a very cost-effective and reliable method of deriving dynamic position now that the GPS satellite constellation is fully in place and operational. Continuous three dimensional position, together with velocity, is available at fast update rates for the videographic requirements of a moving aircraft platform.
      The decrease in size, price, power requirements and complexity of operations associated with airborne videography systems now enables portable, low-cost, simple systems to be realized which can easily be attached to almost any airborne platform. Down on the ground the video data, together with the positioning information, can be televised to a general audience which is often more easily assimilated than more complex satellite vies and map interpretations. Alternately, the video can be applied to a PC, Macintosh or workstation and be easily captured or "frame-grabbed." The captured image, together with its associated coordinate data, can be incorporated into any image analysis or geographic information system.
      In its simplest form the system consists of a nominal vertically mounted video camera and recording system. By using GPS to track the linear features, such as a proposed route, the GPS position and other data is simultaneously encoded into the video signal from the camera and recorded on a conventional video recorder for subsequent review and analysis. An on-board LCD television monitor provides the pilot/operator with a view of the track over the ground being recorded by the video system. Such elementary systems do not make use of DGPS nor gyro stabilized platforms; however this is not to say that they cannot, but that these additions may be used to further refine the accuracy of the end result.
      The system currently in use does not purport to be an accurate mapping tool nor, indeed, to replace conventional mapping techniques. However, for a number of applications which are more concerned with the time, cost and quick-look capability, rather than a product which has been rigidly ortho-rectified offered by other techniques, airborne videography is a practical and efficient route. Two applications are briefly described where airborne videography was used.

THE SYSTEM
The system consists of a medium or high resolution video camera and appropriate lens. The choice of these items will depend upon the altitude and terrain over which the videography is to be conducted and the resolution of the video image and resultant swath width (typically 150 meters to 1 kilometer) required of the captured video.
      The camera is mounted to the outside of the aircraft, preferably looking vertically downwards to avoid parallax errors. There is however merit in mounting the camera or secondary camera looking at the angle slightly forwards such that a more perspective view is achieved and identification of visual references is much easier, particularly with linear topographic objects.
      The video signal from the camera is then combined with data from a conventional GPS receiver by a TELENAV video encoding and mixing unit which encodes the digital GPS data into the vertical interval of the video signal, such that it does not corrupt the picture content and may be recorded on a conventional video recorder mechanism.
      The encoded data is recorded by the on-board VCR and a monitor of the televised scene is provided for the operator. The scene as viewed by the camera may be overlaid in real time with all the relevant GPS parameters and a lat/long to grid (UTM) facility gives full geographic or metric coordinates to the viewer.
      After the operation, the recorded tapes may be reviewed at will and by using the same, or similar, TELENAV unit to decode the data, the overlay of GPS position and other parameters may be shown-pertinent to the frame being displayed.
      For GIS applications the GPS data is output from the TELENAV unit in digital form from the decoded video at the same update rate as received from the originating GPS receiver (typically every second) and, being in conventional RS232 serial format, will interface to any GIS system. This facility, together with any digitized video held in the GIS memory, may be used to analyze or manipulate the videographic material, as required.

APPLICATIONS Urban Mapping and Population Study
Lagos City in Nigeria has a growing population with a variety of water, heating and power requirements. The problem was to assess quickly, and with limited budget, the size of the metropolitan area. The GIS team working together with demographers and town planners were then able to derive an estimate of current population. This would then be used by the utility companies in planning for future needs. Conventional aerial photography was ruled out because of cost and low cloud base-due to the rainy season. It was decided to acquire the most recent SPOT satellite scene (1994). This was used as the map-base. High resolution, georeferenced airborne videography was acquired within two days over selected areas. This together with the SPOT data was used to calculate housing unit numbers and neighborhood area size using ArcView 2. The demographers and town planners were then able to input their data into the GIS and derive an estimate for population size.

Pipeline Planning
An oil company in Columbia, South America needed to build/upgrade a pipeline route from its main producing oil field in the interior, across the Andes to terminate on the north-western coastline. The weather over the planned route ranged from scattered cloud at 3000' agl in the low regions to complete overcast conditions at 1500' agl in the high regions. For environmental and engineering applications a photographic record with location information of the proposed route was required prior to construction. Satellite imagery did not have the resolution nor the appropriate acquisition data. A conventional aerial survey was mobilized but subsequently abandoned due to deteriorating weather conditions. An airborne videography survey was mounted and the attendant positional accuracy and quality-when compared with conventional techniques-produced an acceptable result. The aircraft platform was able to operate below cloud-base in the low region and, using a tracking GPS, record the proposed route. The route over the mountainous regions is to be subsequently recorded using a twin engine helicopter. Selected video frames have been frame-grabbed and incorporated into ArcView 2.

SUMMARY
The use of readily available and economic instruments needed for aerial videography enables rapid response to large area data gathering and is not restricted to the two examples outline above. The value of any video material gathered from a moving platform as provided by aircraft or motor vehicle is significantly enhanced by the addition of position and other parameters related to velocity and altitude, all of which are readily available from GPS.
      The use of a system which combines this data with the video without corrupting the video image has distinct advantages over systems which "burn in" the data and obscure parts of the image unnecessarily. TELENAV (patent pending) combines GPS and other data invisibly within the video signal and, being in robust digital form, enables iterative copies of recorded material to also carry the position data for display or GIS annalysis.
      A combination of data into higher resolution systems can similarly be provided by TELENAV as the data encoded is as accurate as that provided by the positioning sensors employed for the task, such that DGPS or other sytems can be used as the data source, as demanded by the application.

About the Authors:
Roger D. Cooper is the technical director of Navtech Systems Ltd. He may be reached at 703-273-5212 (U.S.) or +44 (0)1858 525454 (U.K.).
Timothy McCarthy is a director of Airborne Videography Ltd. and GIS Mapping International Ltd. in London, England. He may be reached at +44 (0)181 994 4068.
Jonathan Raper is senior lecturer at Birkbeck College and a director of Airborne Videography Ltd. in London, England. He may be reached at +44 (0)1816 316 470.

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