Generating Continuous Surface Probability 

Maps from Airborne Video Using Two 

Sampling Intensities A ong the Video 

Transect 

Dennis M. Jacobs and WWam H. Cooke I11 

Abstract 

Airborne videography can be an effective tool for assessing the 

effects of catastrophic events on forest conditions. However, there is 

some question about the appropriate sampling intensity to use, 

especially when trying to develop correlations with probabilistic data 

sets such as are assembled through the Forest Inventory and Analysis 

(FIA) surveys. We used airborne videography to assess damage to 

forest resources by Hurricane Andrew and the catastrophic February 

8-1 1, 1994, ice storm; but, those efforts were limited by the absence 

of a direct link between video imagery and FIA field plots. In this 

study, we used the 1994 ice storm in northern Mississippi to test bias 

and accuracy at two sampling intensities-14.5- by 14.5-km spacing 

(square) and a 14.5- by 1.6-km spacing (rectangular). Results showed 

that the square pattern resulted in less biased estimates of damage. We 

suggest that this bias would be lessened fbrther if sampling was made 

on the 4.8-ktn sample grid of FIA data points. We assert that bias 

could be eliminated if imagery was acquired directly over FIA plots. 

Keywords: Airborne videography, digital-image files, FIA, global 

positioning system, ice storm, isozone. 

Both portable and easy to use, current airborne videography 

equipment is the ideal technology for quickly assessing 

damage to forest resources following catastrophic events. 

The U.S. Department of Agriculture, Forest Service, 

Southern Research Station, Forest Inventory and Analysis 

(SRS-FIA) research unit uses airborne videopaphy and 

global positioning system (CPS) data to assess such 

damage. SRS-FLA also participates in cooperative research 

projects that effectively use airborne video reconnaissance. 

Airborne videography has been used successfully to assess 

resource damage. Jacobs and Eggen-McIntosh (1 993) 

reported consistent and repeatable methodologies, which 

had been developed following Hurricane Andrew's 1992 

lansall in the Atchafalaya River Basin of southern 

Louisiana. The severe ice storm in northern Mississippi 

provided an opportunity to refine assessment procedures 

(Jacobs 2000), allokng us to increase consistency in visual 

analysis of video samples and automate the geopaphic 

infomation system (CIS) process used for mapping damage 

severity and extent. According to the 199 1 State inventory 

(Visage and others 1992), the hurricane study area was 1.7 

million ha and contained 729,000 ha of timberland. By 

comparison, the ice storm study area was 1.5 million ha and, 

according to 1994 inventory data (Faulkner and others 1993, 

Hartsell and London 1 995), included 855,000 ha of 

timberland 

For both studies, we captured digitally some selected video 

f?ames from continuous video recordings and visually 

interpreted damage severity. Damage classes and associated 

video-fi.ame GPS coordinates were mmbined as point data 

age-severity isolines. The GIs polygon 

coverage that resulted was used to extract field-plot 

infomation from associated FL4 survey data. Plot data were 

compiled to generate a timber volme damage report by 

&age-class variable and species group for each study 

ma. 

Using airborne videography, we recorded a continuous 

transect of aerial imagery and acquired an almost unlimited 

density of sampling points along the flight line. However, 

cost vs. benefits played a major role in determining the 

number of flight lines, and the distance between flight lines 

for recording airborne videopapby limited both studies. 

Our objective was to determine whether the bias in 

restm@ar spacing of point data (I 4.5 by 1.6 km) could be 

overcome by reselecting data points along the flight lines 

equal to the distance between flight lines (1 4.5 by 14.5 h). 

Materials and Methods 

The airborne video system is used to acquire vertical aerial 

video imagery and identify images by geographic 

coordinates. The equipment is easily traflsportable and can 

be installed in an aircraft on the same day a video mission is 

flown, The system includes four main parts: (1) video camera 

head, (2) color video monitor, (3) CPS receiver, and (4) video 

recording unit containing a graphics computer. The video 

camera head contains a 213-inch format (1 6.93 mm), chargecoupled device (CCD) as the imatE;lng plane (8.8 by 6.6 

An 1 1 - to 66-mm zoom lens with auto-iris is attached to the 
camera head, and the camera head's overall weight is 1.25 kg. 

The video-recording unit includes an 8-mm videocasgtte 

recorder arrd gaphies computer, which can generate text 

onto the video imgery; e.g,, CPS coordinates, altitude, and 

date md time. The recording wit also has a keyboard for 

g the mission as welt as power 

connections for all peripheral devices. 

A flying altitllde of 660 m and ma zoom of 66 mm for 

the focal length of the camera lens provided a video-image 

footprint of 88 by 66 m (0.58 ha) for each sample location. 

Video flight lines were sprteed at 14.5 km and oriented 

pevndicdar @o~WSouth) to the storm track. We 

subsampled continuous ve-tZical aerial video imagery (fime 

capturd) along each flight line at 1.6-km (1 mi) and 14.5-km 

(9 mi) intervals for sparate interpretation and analysis at 

both low- and kgh-sampling densities. We used GPS 

coordinates, encoded onto the video imagery in-flight, to 

mme each ofthe fiame-captwed, digiaf-hage files. File 

nmes wwe istput as gmgaplrric cosr&nates, creating a G!S 

poirrt coverage of the video sample point locations and 

associated hmage-condition attributes. We chose sample 

pokt spacing for analysis at low (14.5 by 14.5 km) and high 

(1 4.5 by 1.6 km) densities to illusbrate the difference bdween 

sqwe-@d and rectanplar-ad sampling and mapping 

hlctionality. 

We intevreted video frames for hardwood and pine basal 

area and lyouped our visual interpretations into one of four 

percent-hardwood categories. For each species group, we 

classified damage conditions separately; pines were more 

susceptible to stem damage from ice than hardwoods 

(blverson and Guldin 1 995). Ordinal darnage severity 

classes incluhd: 0 = none, 1 = light, 2 = moderate, and 

3 = severe damage. We interpreted all video fiames by crossvalidation ammg flight lines and fiames recording similar 

We chose kc/Info's "SPLINE'%ction within the GRID 

program to calculate isolines from the point data. Our results 

were continuous sdace probability maps showing damageseverity classes. We produced maps for low- (1 4.5 by 14.5 

km) and high- (14.5 by 1.6 km) &me-sampling densities. 

Results 

Fipes 1 and 2 illustrate the continuous srarface probability 

maps from the ""SLW firnctim for the two sampling 

densities. An EastNJest measurement bias is obvious in 

Figure 1-Damage-severity isozones generated from high smple rate of 1.6 km by 14.5 km. An indicator of forest 

resources timber damage. 
Figure 2-Damage-severity isozones generated from low sample rate of 14.5 km x 14.5 km. 

figure 1. This bias is generated when the "SPLINE" hction 

interpolates isolines within and around the data points 

spaced at 14.8- by 1.6-km density. Wen an isoline is placed 

around similarly valued data points on adjacent flight lines, it 

must be drawn over 14.8 kin of open space before conto~ng 

between two data points spaced at 1.6 lam along the flight 

line. An additional problem is gaps of no-data values along 

the flight line. Although continuous transects of video 

imagery were recorded, there were several places along the 

flight lines where no timberland was recorded for up to 13 

lam. A gap ofthis size would create a block of missing 

tiniberland data as large as 30 by 13 h. 

Figures 3 and 4 illustrate FIA plot locations on surface 

probability maps, The low-damage isozones changed 

slightly in figure 4 because of the lower sampling intensity. 

The high-damage isozones had a greater change, both in size 

and in location, which resulted in plots receiving different 

damage assessment values. 

Suggested preflight pt ng requiremen% of 4.8 km (3 mi) 

fli&t-line centers necessary to coincide with FM's 

systematic inventory grid would help eliminate this bias. 

Administrative decisions and cost benefits concerning flight 

time for this study preempted the plan to space flight lines at 

4.8 h. EEofis at estimating timber losses and damage 

affecting growth and mortality will help us desribe ow 

objectives for projects like these. 

Discussion and Gonclusicbns 

Automated isoline calcula~on progams are includecl w3.th 

most GIs soffware and provide a major improvement over 

hand &awing of damage-seve~ty polygons. Such progams 

reduce analyst subjectivitqP arid &prove repeatability of 

results. 

The greater sampling density of ~ctangulr-ur-spaced sample 

points creates a visual bias because there are more samples 

NortWSouth than EasWest. The low-sample density of 

regularly spaced sample points removes the effect of Easti 

West bias, but it does so at the expense of reduced 

interpolation precision. Conclusions from this study 

strongly susest that 4.8-b flight-line centers will be 

necessary to synchronize this type of maIysis with the 4.8 
Little or none 

Light 

Moderate 

Heavy 

Severe 

Figure 3-Damage-severity isozones generated from high sample rate of 1.6 km x 14.5 km. FIA plot locations shown 

within the damage zones. 

Figure 4-Damage-severiry isozones generated from low sample rate of 14.5 km x 14.5 km. FIA plot locations shown 

within the damage zones. 
by 4.8 km systematic spacing of FIA plot data. Lack of field 

verification of damage-severity classes was a problem in this 

study, To ensure optimum field verification, future studies 

would also benefit from aerial imagery captured directly over 

FIA plots. 

The airborne video system developed and used by SRS-FIA 

has been a valuable tool when rapid responses to natural 

catastrophic events were necessary. The combination of 

wind and ice damage (ice storm), or wind damage alone 

(hurricanes) can be characterized spatially through 

interpretation of aerial video. GPS coordinates encoded onto 

the video facilitate development of a GIs for an affected area, 

GIs data, coupled with current databases of forest resources, 

enable analysts to develop estimates of damage soon after 

the storm event. 

Airborne video is increasingly used as a monitoring tool. 

SRS-FLA has been involved in cooperative projects using 

GPS and airborne video to provide vegetative ancillary 

information for satellite imagery classifications in Mexico 

and for mapping and monitoring natural resource changes in 

western Africa (Wood and others 1995). Cooperative efforts 

of SRS-FIA and the Indonesian Ministry of Forestry, as well 

as the Wagenengin Agricultural University in The 

Netherlands, use frame-captured aerial videography to 

facilitate interpretation of airborne radar imagery over the 

Jambi province of Sumatra, Indonesia. We have scheduled 

hands-on interpretation of radar imagery and aerial 

videography for future projects. This is an exciting 

opportunity for technology transfer between countries 

interested in monitoring the condition of forest resources 

worldwide. 

Literature Cited 

Faulkner, Joanne L.; Miller, Patrick E.; Hartsell, Andrew J.; 

London, Jack D. 1993. Forest statistics for north Mississippi 

counties-1994. Resour. Bull. SO-1 80. New Orleans: U.S. 

Department of Agriculture, Forest Service, Southern Forest 

Experiment Station. 41 p. 

Halverson, Howard G.; Guldin, James M. 1995. Effects of a 

severe ice storm on mature loblolly pine stands in north 

Mississippi. In: Proceedings of the eighth biennial southern 

silvicultural research conference; 1994 November 1-3; Auburn, 

AL. Gen. Tech. Rep. SRS-1. Asheville, NC: U.S. Department of 

Agricuiture, Forest Service, Southern Research Station: 147-1 53. 

Hartsell, Andrew J.; London, Jack D. 1995. Forest statistics for 

Mississippi counties-1994. Resour. Bull. SO-190. New Orleans: 

U.S. Department of Agriculture, Forest Service, Southern Forest 

Experiment Station. 89 p. 

Jacobs, Dennis M. 2000. February 1994 ice storm: forest resource 

damage assessment in northern Mississippi. Resour. Bull SRS-54. 

Asheville, NC: U.S. Department of Agriculture, Forest Service, 

Southern Research Station. 

Jacobs, Dennis M.; Eggen-McIntosh, Susan. 1993. Airborne 

videography and GPS for assessment of forest damage in 

southern Louisiana from Hurricane Andrew. In: Proceedings of 

the IUFRO conference on inventory and management techniques 

in the context of catastrophic events; 1993 June 21-24; 

University Park, PA. [Published in electronic format.] 112 p.] 

http:/lwww.srs.fs.fed.uslpubslja~jajacobsOOl .pdf (1 1 September 

2000). 

Vissage, John S.; Miller, Patrick E.; Hartsell, Andrew J. 1992. 

Forest statistics for Louisiana parishes-1991. Resour. Bull. SO168. New Orleans: U.S. Department of Agriculture, Forest Service, 

Southern Forest Experiment Station. 65 p. 

Wood, Eric C.; Tappan, G. Gray; Jacobs, Dennis M. 1995. 

Monitoring Senegal's natural resources using airborne videography. 

In: Proceedings of the fifteenth biennial workshop on color 

photography and videography in resource assessment; 1995 May 

1-3; Terre Haute, IN. Bethesda, MD: American Society for 

Photograrnmetry and Remote Sensing. USA. 315-325. 

C. U.S. PRItWINZ OFFICE: 2000-565-019/00033 
Jacobs, Dennis M. and Cooke, William H., 111.2000. Generating continuous surface 

probability maps from airborne video using two sampling intensities along the video 

transect. Res. Pap. SRS-22. Asheville, NC: U.S. Department of Agriculture, Forest Service, 

Southern Research Station. 5 p. 

Airborne videography can be an eEective tool for assessing the effects of catastrophic events 

on forest conditions. However, there is some question about the appropriate sampling intensity 

to use, especially when trying to develop correlations with probabilistic data sets such as are 

assembled through the Forest Inventory and Analysis (FIA) surveys. We used airborne 

videography to assess damage to forest resources by Hurricane Andrew and the catastrophic 

February 8-1 1, 1994, ice storm; but, those efforts were limited by the absence of a direct link 

between video imagery and FIA field plots. In this study, we used the 1994 ice storm in 

northern Mississippi to test bias and accuracy at two sampling intensities-14.5- by 14.5-km 

spacing (square) and a 14.5- by 1.6-km spacing (rectangular). Results showed that the square 

pattern resulted in less biased estimates of damage. We suggest that this bias would be lessened 

Wher if sampling was made on the 4.8-km sample grid of FIA data points. We assert that bias 

could be eliminated if imagery was acquired directly over FIA plots. 

Keywords: Airborne videography, digital-image files, FLA, global positioning system, ice 

storm, isomne.