Underwater Digital Stereo-Observation Technique for Red Hydrocoral Study

A.K. Chong and P. Stratford

Abstract provide valuable insights into the recruitment and population A linear regression model is used to determine the size of d3~"amicsof the Fiordland population. Knowledge gained will correction required for stereophotogrammetric measurement also provide into the biology of many ~ffshorespecies, from photographs taken in an underwater environment. The which are often adversely affected by c~nu~~ercialfishing model utilizes the stereophotogrammetric distance measure- tice~(Probert et a1.S 1997)-Deep-sea trawls frequently bring up ment of an object from a set of object-space control points. A deep-watercorals~ecies.Unfortunatel~j thelong-temeffecton correction factor is applied to the stereophotogrammetric these be studied measurement to obtain the corrected measurement. The technique was used to study the growth of an endemic species Exllng Coral Growth Measurement Techniques of hydrozoan coral, Errina novaezelandiae, in Doubtful Sound, A number of techniques are available to biologists for measur- Fiordland, New Zealand. The results show that the technique ing growth in corals. Vital staining with alizarin or calcein, X- has a measurement accuracy of t 1.2 mm at the 95 percent ray radiography, and single-framephotography are some of the confidence level. We found that these hydrozoan corals have more common techniques. A brief discussion of these tech- a high erosion rate, presumably due to grazing, which was not niques and their advantages and disadvantages is provided reported previously. below. Introduction Vital Staining Close-range stereophotogrammetric techniques have been used successfully to determine the size of different species of ma- Alizarin and calcein are two cost-effective chemical markers, rine vertebrates in their natural environments. Such species in- which are non-toxic and are accurate for marking the growth of clude fish (Klimley and Brown, 1983; Harvey and Shortis, coral in its natural environment (Lamberts, 1978; Le Tissier, 1996;van Rooij and Videler, 1996),whales (Riither and Adams, 1988; Rowley and MacKinnon, 1995; Ward, 1995). To mark a 1984; Cubbage and Calambokidis, 1987; Ratnaswamy and colony at the start of a monitoring period, a plastic bag is placed Winn, 1993; Dawson et al., 1995), and Hector's dolphins over the entire colony and concentrated alizarin or calcein so- (Brgger and Chong, 1999; Chong and Schneider, 2001). Litera- lution is injected into the bag through a special opening (Le Tis- ture review shows that Done (1981) gave the first discussion on sier, 1988). The bag is removed after several hours, leaving a the use of stereo photography for coral reef ecology study. In chemical stain at the site of active calcium carbonate the present research, a digital stereophotogrammetric tech- deposition. nique is applied to an underwater sessile organism, the red hy- To measure the amount of growth in the colony, branches drocoral Errina novaezelandiae. are removed from the colony after a period of time and the skel- Rosencrantz (1971) and Torlegard and Lundalv (1974) used eton is cleared of organic material using bleach (Lamberts, underwater photogrammetric techniques to determine the un- 1978;Ward, 1995).The chemical stain can then be viewed and derwater accuracy of stereoscopic observation. According to measured using epi-fluorescence microscopy or with high in- their finding, an underwater photogrammetric technique may tensity blue or ultraviolet light in combination with an inter- be suitable for measuring growth in hydrozoan corals. Recently, ference filter that transmits 514-nm wavelength light (Wilson Fuller and Faig (1995) and Shortis et al. (2000) discussed asim- and Beckman, 1987; Rowley and MacKinnon, 1995). The ilar technique for mapping the sea floor and for shellfish sur- measurement accuracy is estimated to be around +0.10 mm at veys, respectively. the 95 percent confidence level, depending on the equipment Hydrozoan corals (hydrocorals) are typically deepwater, (e.g., microscope or calipers) used for measurement. temperate species and are not generally found on mainland The main disadvantage of this technique is that the entire coastlines. Hydrocorals can be found in close association with colony must be sacrificed or a large number of branches must scleractinian corals in tropical environs, but are more typically be removed in order to determine the growth rate. Therefore, found on offshore seamounts and submarine ridges. However, repeated measurements of the same branch or colony over time an endemic species of hydrocoral, E. novaezelandiae, is found are not possible. In addition, any erosion or grazing that may in the fiords of Fiordland, southwestern New Zealand. occur can remove the chemical stain and hence growth cannot Much of the current knowledge of hydrocoral biology has be measured. been deduced from small, one-of-a-kind samples collected from deep water using dredges. Increased tourism (Grange, 1990; Miller, 1995)and the possibility of souvenir collectors in the Fiordland region heighten the need for research on the biology of this endemic species of hydrocoral. Accurate informa- Photogrammetric Engineering & Remote Sensing tion on the reproduction and growth of E. novaezelandiae will Vol. 68, No. 7, July 2002, pp. 745-751. 0099-lll2/02/6807-745$3.00/0 School of Surveying, University of Otago, 304 Castle Street, 0 2002 American Society for Photogrammetry Dunedin, New Zealand ([email protected]). and Remote Sensing

PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING lulv 7002 745 L X-Ray Radiography Branches or large portions of a coral colony are removed and the organic material is dissolved in bleach. Washed and dried samples are sliced and X-rayed using standard medical X-radiography equipment (Buddemeier and Maragos, 1974; Logan and Tomascik, 1991; Peirano et al., 1999; Lough and Barnes, 2000). Examination of the density levels on the X-ray negatives identifies any growth banding patterns. If the periodicity between bands can be determined (e.g., using vital staining), growth rates over relatively long periods can be determined (Buddemeier,1978; Logan and Tomascik, 1991).The measurement accuracy of this technique is expected to be approxi- mately 0.1mm at the 95 percent confidence level. Again, the disadvantage of this method is that it requires destructive sampling.

Single-Frame Underwater Photography Figure 1. The underwater stereocamera system. Note the control frame and the shutter control dev~ce. Single-frame photographs are often taken with a scale placed beside the photographed colony (Miller, 1998).This is a popu- lar technique because specialized equipment is not needed and a specialist is not required to measure the growth between ep- Camera Lens Calibration ochs. However, the accuracy is very difficult to determine be- Cameras used for this project had to be calibrated underwater, cause the angle of the photography and the scale position var- in addition to the standard calibration, to achieve the required ies significantly between epochs. Some researchers believe that mean positional standard error of 21.0 mm that had been the measurement accuracy varies between 2 2 and 26 mm at achieved by Fryer and Fraser (1986). Standard non-metric the 95 percent confidence level (Ben-Zionet al., 1991; Miller, camera calibration is well documented (Fryer, 1989; Beyer, 1998). 1992; Peterson et al., 1993; Fraser and Edrnundson, 1996; Shortis et al., 1996; Clarke and Fryer, 1998). This process in- cludes the determination of the principal point of autocollima- Objectives of This Paper tion, the principal distance, the radial lens distortion parame- The vital staining and X-ray radiography techniques are highly ters, and in some instances the dynamic fluctuation. Published accurate. These techniques are, however, not suitable for hy- works on underwater camera calibration and underwater drocoral study in Doubtful Sound. Apart from being a protected stereo-photogrammetric systems can be found in Pollio (1971), species, the limited number of Errina novaezelandiae colonies Rosencrantz (1971),Fryer and Fraser (1986),Li et al. (1996), means that any substantial removal of branches or whole colo- and Shortis et al. (2000).Fryer and Fraser (1986)authored the nies may have detrimental effects on the Doubtful Sound pop- most recent article on underwater camera calibration tech- ulation. An opportunity, therefore, exists to develop a non-de- niques that are suitable for this research. These researchers structive technique that will yield detailed and accurate used a plumbline test frame and a self-calibrating bundle ad- growth information on a threatened and otherwise inaccessible justment to determine the various camera lens parameters. In coral species. Stereophotogrammetry is considered a viable their lens calibration, Fryer and Fraser achieved a mean posi- technique because it is non-destructive and colony disturbance tional standard error of ?l.O mm. They found that refraction at is minimal. the waterllens surface causes the principal distance of a camera The main objective of this paper is to present an underwa- immersed in water to appear to increase by a factor of 1.34, a ter, high-accuracy, close-range digital stereo-observation tech- common value for the refractive index of water. nique. A correction scale factor is used to improve the measure- We constructed two sets of test-fields using rectro-reflec- ment accuracy in the underwater environment. The second tors for the underwater and in-air (standard) camera calibra- objective is to show how we apply this technique to the study tion, respectively. The underwater test-field consisted of 10 of linear extension rates in the endemic hydrozoan coral, Er- rows by 12columns of 3-mm-diameter rectro-reflectors at a 50- rina novaezelandiae, in Doubtful Sound. mrn spacing (Figure 2) while the standard test-field consisted of Special features of the close-range photogrammetry system 10 rows by 12 columns of 1-mm-diameter rectro-reflectors at a are outlined and discussed. For example, a scale factor based 50-mm spacing. Rectro-reflectors of both test-fields were coor- on the distance to the object from the photogrammetric control dinated using Australis, a state-of-the-art bundle adjustment frame is used to improve the measurement accuracy of the un- software, a Kodak 290 digital camera and a 750-mm BRUNSON derwater stereoscopic measurements. Instrument Invar scale bar with HUBBS rectro-target attach- ment. The mean positional standard error of the coordinate was Underwater Stereo-Camera System Design and Calibration 20.018 mm. In the standard calibration, the Nikonos V cameras were Design of Stereocamera System pre-set to focus at 800 mm (depth of field from 550 to 1050 mm). Figure 1shows the configuration of the stereocamera system. Each camera took a set of four convergent photographs of the Two off-the-shelf Nikonos V underwater cameras (Nikon Nik- control targets at a 800-mm object distance using slide film. kor 35-mm lenses) were mounted 250 rnrn apart on an alumi- The slides were scanned at 2500 dpi. The Australis bundle ad- num bar. A purpose-built device was developed to activate the justment software was used to compute the calibration param- camera's shutters simultaneously. A set of stainless steel rods eters from the digital images. Each camera was calibrated five was used to fix the object-space control frame at a distance of times in order to evaluate the dynamic fluctuation of the lens 800 mm from the cameras. A pair of strobe lights that fire simul- parameters. taneously were fitted to the system to provide the lighting for In the underwater calibration, the underwater test-field deep-water photography. was placed at a depth of 2 m in Portobello Marine Laboratory

746 jury 2002 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING - -

Figure 2. Underwater system calibration. Note the 100 tag, which shows the distance of the test field from the control frame.

holding tanks. Again, the Nikonos V cameras were pre-set to fo- front of the coral. Initial tests showed that there was a small er- cus at 800 mm. Each camera took a set of four convergent pho- ror in the stereo measurement when the control frame was tographs of the control targets at an 800-mm object distance us- placed at a short distance in front of the measured object. Con- ing slide film. The cameras were calibrated five times to sequently, it was necessary to study the effect of a change in evaluate fluctuation of the lens parameters underwater. object distance in addition to the evaluation of the system in an underwater environment. To carry out this test, we photo- Owthe&b Camera Lens Calibration graphed the underwater test-field at various distances from the Both Nikonos V underwater cameras were regularly used for control frame,which was fixed to the camera-mounting frame other underwater research projects between epochs of hy- (Figure 1).That is, the object distance to the test-field was set at drocoral photography. Consequently, the initial set of com- a minimum of 800 mm to a maximum of 1050 rnm at an incre- ~utedlens Parameters could have changed forsubsequentep- ment of 50 mm (Figure 2). Five sets of independent stereo-pho- ochs of hydrocoral ~hotogra~h~.An on-the-job calibration tographs were obtained for each object distance. All the slides could detect changes in the lens Parameters as a result of fie- were scanned at 2500 dpi and each stereo-pair of photographs Pent rough handling of the camera, a change in water Pressure was oriented in a digital stereo-workstation. In each set, a mini- at the study site, or a change in the clarity of the water during mum of six three-dimensional vectors between rectro-reflec- hydrocoral photography. A detailed discussion of the on-the- tors on the test-fields were measured and evaluated. job calibration technique can be found in Alkinson (1996) and Karara (1989). Results and Analysis of System Evaluation To carny out the task, the control frame, which is shown in Both left and right camera lenses were calibrated using the Figure 1,was removed from the stainless steel holding rods at standard camera calibration and underwater camera lens Cali- the required water depth. The control frame was placed on the bration techniques. Table 1 shows the calibrated parameters of sea floor. Aminimum of two convergent photographs (aroll of the left camera lens. The principal distance of the lens changes 30 degrees from the vertical is sufficient)of the control frame from 35-19 to 46-93 However, other parameters re- was taken with each camera (see Slama (1980),p. 837, for con- main very much- the same. It must be noted here that the param- vergent ~hotogra~h~).To compute the lens Parameters of a eters vary considerably when the clarity of the seawater is not camera, the corresponding convergent photographs were ma- the same between testing epochs. On-the-job camera lens cali- lyzed using the Australis camera calibration software. Because bration is essential for all high accuracy photogrammetric there were very few control points on the control frame, only measurements. critical parameters were computed in the bundle adjustment. Tests showed that only the focal length requires re-calibration. Resun of the Swem Evaluation Other lens Parameters such as lens distortion Parameters and Table 2 shows the result of the system evaluation in seawater. principal points offsets did not change significantly between The ratio was obtained by dividing the difference between the epochs of the hydrocoral photography. measured length and the true length by the true length. The "mean ratio and sigma" were computed using 18 sets of meas- Object Space Control for Undemater StereoPhatography urements. The "zero object distance" row shows the accuracy I A rigid 250- by 300- by 25-mm aluminum frame was con- of the stereo-imaging system. In Table 2 this value is 0.003 f- structed (Figure 1).Bolts of various lengths (horn 10to 30 mm) were fastened to the surface of the frame. Rectro-reflectors were placed on the end of each bolt. The procedure used in determi- TABLE1. STANDARDAND UNDERWATERCAMERA LENS CALIBRATION nation of the test-field coordinates, discussed elsewhere in the paper, was used to establish the coordinates of the rectro-re- Calibration c (mm) Xp (mm) Yp (mm) K, (mm) K, (mm) flectors on the object-space control frame. Standard 35.19 -0.48 -0.7 -6.3E-5 4.5E-8 ?6.9E-2 k1.2E-2 +1.3E-2 ?1.7E-5 26.6E-8 Stemcamem System calibration and Evaluation Underwater 46.93 0.54 -0.5 -7.63-5 -8.2E-8 To avoid damaging the coral during photography, it was neces- 28.43-2 21.9E-2 +2.2E-2 k2.73-5 k9.4E-8 sary to set the control frame at a distance of 50 mm or more in

PHOTOGRAMMETRIC ENGINEERINGa REMOTE SENSING llrly 2007 747

L TABLE2. SCALEFACTOR FOR VARIOUSOBJECT DISTANCES (1) Determine the average distance from the control frame to the coral using the measured distances or z values of the start Object Distance Mean Ratio and end of a branch. The distance z of the control frame has (mm) and sigma Linear Regression a datum of zero. That is, the distance z of the coral measure- 0 (zero) 0.003 2 0.001 s = k + k, (x) ment has a negative value. 50 0.038 + 0.005 k = -1.473-03 (2) Obtain scale factors by substituting the variable xin the follow- z s = 100 0.088 5 0.009 kl = 8.653-04 ing equation with the measured distance value: k + 150 0.119 + 0.005 RMS = 5.44E-03 kl(x) where s is the scale factor, k is a constant, k1 is the z 200 0.171 2 0.021 Where s = mean ratio and coefficient of x, and x is the average distance value. The k 250 0.220 + 0.023 x = object distance in mm. and k, are independent variables of the linear regression equation. (3) Add 1.0 to the computed scale factor. A factor of one is added because the measured length is always longer then the corrected length. This undertaking is apparent in Step 4. 0.001. To work out the accuracy of the system, it was necessary (4) Divide the measured length (or coordinates) by the scale factor to determine the maximum length of a branch, which must be in Step 3 to obtain the corrected length. measured in order to compute the growth or linear extension of hydrocoral. Test samples showed that the maximum measured length of a coral branch was 128 mm. Multiplying this length by Hydrocoral Research Project The site selected for the study was the northern side of Elisa- the mean ratio and sigma of 0.003 2 0.001 gave 0.4 2 0.1 mm. The value is less than the required mean positional standard er- beth Island in Doubtful Sound (Figure 4). Fifteen Errina novaezelandiae colonies in a range of sizes (120 to 220 mm maximum ror of 2 1.0 mm, which was discussed elsewhere in the paper. The majority of the measured lengths for this project were on width) were selected at depth of 16 to 18 m. Each colony was the order of 40 mm or less. tagged with an identification number. Initial stereo-photographs were taken in November 1998 and again in November Significance of the Ratio 1999. As discussed elsewhere in the paper, there was a small change The cameras were pre-set to focus at an object distance of in the ratio for stereo-measurement when the control frame 800 mm. The control frame was placed at 50 mrn to 150 mrn in was placed at a short distance in front of the measured object. front of the faqade to be photographed (Figure 5). Where possi- To evaluate the effect of the change in the ratio value, a maxi- ble, the same fa~adewas photographed for all epochs. This mum measured length of 128 mm and an average object dis- made it easy to identify the branches used in the monitoring. tance of 82 mm were used in the computation. Using the ratios On each visit to the site, replicate pairs of stereo-photographs in Table 2, the computed length of a 128-mm-long branch pho- were taken. tographed at an object distance of 82 mm is 119.9 mm. An 8.1- Digital stereo-orientation was carried out in an off-the- mm difference between the measured and computed length ex- shelf digital stereo-plotting workstation. The interactive pro- ceeded the required measurement accuracy of +1 mm by 7.1 gram requires an interior orientation and observation of the fi- mm. ducial marks [e.g., etched markers on the side of the film To correct for the error, it is necessary to compute the cor- mount or corners of the film mount of a non-metric camera). rection. Column 3 in Table 2 shows the linear model used to The calculated focal length from the on-the-job camera lens cali- model the size of the error based on the object distance. The re- bration was used in the interior orientation. The computed ex- lationship between the correction scale factor and the distance terior orientation parameters from the initial stereo-camera of the test-field &om the control frame is depicted in Figure 3. system calibration were used as the initial approximations in Based on the RMS of the linear regression shown in the same table, the error for a measured length of 128 rnm is 0.7 mm. As a result of the correction, the underwater technique can achieve the accuracy required for the project.

Application of the Conection Factor To obtain a correction for any measured length, the following procedure was adopted: New Zealand

42O

y = 0.0009x 0.250 - R' = 0.9951

45O

0 50 100 150 200 250 300 Distance of Object (mm)

Figure 3. A correction scale factor chart showing the rela- tionship between the correction scale factor and the dis- Figure 4. Study site and habitat of the red hydrocoral of the tance of the test-field from the control frame. South Island, New Zealand.

748 July 2002 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING Figure 5. A pair of stereophotographs of a red hydrocoral colony. Note the identification tag on the colony.

the relative and absolute orientation. The digital stereo-plot- negative values, caused by grazing or other physical processes, ting workstation software determined the optimum orientation cannot be determined using the calcein technique. The ranges parameters using the control points of the control frame. We of linear extension measurements for both techniques are rea- found that the number of control points on the control frame sonably comparable (Table 3). The smaller mean and standard was adequate for accurate stereo-orientation of all the stereo- error for the calcein technique is attributable to a large number model. Subsequently, the stereo-models were digitized to ob- of small measurements from one colony. Consequently, the tain the extension of the hydrocoral. Each pair of stereo-photo- standard error appears to be smaller than for the stereo tech- graphs was analyzed in the digital photogrammetric plotting nique. Measurements from a larger number of colonies may system. help alleviate the uneven distribution pattern observed in the Seven randomly selected colonies (from the 15 photo- calcein technique. graphed) were used in the photogrammetric study, while only Also shown in Table 3 are the stereo-photogrammetrically three colonies were available for the calcein study. For the pho- derived negative and net values for Enina novaezelandiae lin- togrammetric analysis, the changes in length (Thd - TMtid)for ear extension rates. Some colonies show a substantial amount seven to ten branches per colony were measured, whereas all of erosion or grazing, with decreases in branch length of up to stained branches over a 10-month period were used for the 15.6 rnm. In the field, small gastropods were observed to live on calcein analysis. Linear extension rates were standardized to colonies of E. novaezelandiae, but their impact is not known. annual extension rates. Corallivorous fish may also graze on the branches of the The linear extension rate for both techniques was com- colonies. puted by taking the mean of individual branch measurements. Figure 6 shows the mean positive growth in comparison to The standard deviation of the difference between duplicate the mean net growth at the Elizabeth Island site. In all colonies measurements for the same branch was used as an indicator for the accuracy of each technique. The net extension rate for the photogrammetry technique was obtained by combining the 20 - # positive and negative values, and the computed values were Positive Growth 8 Net Gmwlh used to calculate the mean and standard deviation. The age of 3 colonies was computed by dividing the maximum height of a -f- 15 - 6- 8 colony by the overall mean extension rate. E 6 -E lo- Resun of the Hydrocoral study e Table 3 shows the results of a study using the calcein technique and the stereo-photogrammetrytechnique. Only positive ex- tensions can be compared between the two techniques because r5, 0 TABLE3. COMPARISONOF THE MEANBRANCH EXTENSION RATES IN ERRINA m I NOVAEZELANDIAEFOR THE CALCEINAND THE STEREOPHOTOGRAMMETRIC MEASUREMENTTECHNIQUES. E~NSION RATES WERE STANDARDIZED TO A PERIOD -10 - OF ONE YEAR.n = NUMBEROF INDIVIDUALBRANCHES MEASURED I 4 5 7 8 11 14 Colony Growth Mean (SD) Raw Accuracy Measurement N (mm yr-') (mm yr-') I~I~I Figure 6. Mean (2SE) annual branch extension rates for Calcein (positive) 43 3.1 + 0.6 0.2 to 18.2 f0.13 colonies of Errina novaezelandiae. Numbers above each Stereo (positive) 39 8.0 f 0.9 0.0 to 21.9 20.6 column indicate the number of branches measured per Stereo (negative) 23 -6.2 t 0.1 -15.6 to -0.4 20.6 colony. Stereo (net) 62 2.7 2 1.1 -15.6 to 21.9 20.6

PHOTOCRAMMETRIC ENGINEERING & REMOTE SENSING luly 2002 749 the net growth of the branches was considerably less than the one epoch to the next. The water environment at the study site mean growth, with three of the seven colonies showing negative changes constantly. However, our study using a control frame net extension rates, indicating overall colony shrinkage. The for absolute orientation of the underwater stereomodel study also showed that most colonies at the Elizabeth Island showed that the control frame reduced or eliminated a signifi- site were relatively young [Table 4). cant amount of the error introduced by this factor.

The Effect of Slightly Different Photographic Angles Potential Madma Applications We studied the possibility of introducing measurement error as We believe that the developed high accuracy underwater pho- a result of photographing the hydrocoral at slightly difference togrammetric technique is suitable for many applications that angles between epochs. We used the same diver for all the pho- require accurate and long term monitoring. In the present study, tography, but the diver was unable to determine the exact posi- stereo-photogrammetry has been applied to measure linear ex- tion and direction of photography from previous epochs. To tension of a branching hydrocoral. With only slight modifica- conduct the study, the diver repeated the photography at a tion to the described technique, overall colony growth could slightly different position and pointing direction each time. be estimated more accurately using changes in either volume or The mean difference between measured branches was -0.4 area. Other potential applications for underwater stereo-pho- 50.6 mm (n = 13,range: -1.5 to +0.5 mm), indicatingthat the togrammetry include repeatability of measurements on stereo-photographs-taken at a slightly different angle is within the required accuracy. Monitoring of the effects of natural and anthropogenic environmental changes on benthic organisms; Quantitative analysis of population dynamics, growth, and pro- Diimion ductivity, particularly in slow growing and sessile inverte- An accurate underwater stereo-photogrammetric technique brates; and was developed to obtain the linear growth parameters for the Detailed morphometric analysis of individuals and populations. endemic hydrocoral, Enina novaezelandiae, in Doubtful Sound. In this section we discuss a few particulars relating to the developed technique. We have developed and evaluated a high accuracy photogram- Calceln or CWange Phat- metric technique, which can provide accurate branch length The calcein technique is not considered suitable for coral measurements for corals in & underwater environment. we branch extension studies because it can only detect positive found that the technique is efficient because a large number of growth. That is, there is no provision for the estimation of the colonies can be photographed in short periods of time. In addi- effects of grazing or erosion on branch growth. Additionally, tion, we found that stereo-photographs are excellent for the the calcein technique requires destructive sampling, which in identification of individual branches from the hundreds of the case of Errina novaezelandiae, and other rare or protected branches that may occur in a single colony. Subsequently, species, is considered inappropriate. Consequently, the stereo- stereo-photographyallowed us to ascertain that the endemic photogrammetry technique is the most desirable technique for hydrocoral, Enina novaezelandiae, has a relatively high ero- the monitoring of branch extension rates in E. novaezelandiae sion rate due to physical damage and grazing, which had not within Doubtful Sound. been reported previously. S~~orM-~phwmJw Acknowledgments It is a well-known fact that multi-convergentphotography gives Research funds for this study came from a University of Otago the optimum accuracy in close-range photogrammetry. Under- Research Grant, PAD1 Project AWARE, USA and Fiordland water photographs suffer from optical degradation, reducing Travel Ltd, New Zealand. The authors thank the people who of- the ability to select an identical point from two or more conver- fered constructive criticism on this manuscript. gent photographs. Light attenuation, occlusions (Fuller and Faig, 1995), and sedimentation are some examples of causes of the degradation of the underwater imagery. Without the aid of a stereoscopic view of the coral branches, it would be close to impossible to digitize the position of the various branches re- Atkinson, K.B., (editor), 1996. Close Range Photogrammetry and Ma- peatedly and accurately. chine Vision, Whittles Publishing, Bristol, United Kingdom, 378 p. OntheJob Cmra Lens Callbratkn Ben-Zion, M., Y. Achituv, N. Stambler, and Z. Dubinsky, 1991. A photographic, computerised method for measurements of surface area As discussed elsewhere in the paper, water pressure and clarity in Millepom. Symbiosis, 10:115-121. of the water can affect the computed focal length of a camera in an underwater environment. On-the-job calibration showed Beyer, H.A., 1992. Geometric and Radiometric Analysis of a CCD- Camem Based Photogmmmetric Close-Range System, Mittei- that the Nikonos V underwater camera lens focal length lungen Nr. 51, Institut ftir GeodLie und Photogrammetrie an der changed with the depth of the water. The computed focal length Eidgentissischen Technischen Hochschule, Ztirich, Switzerland, changed from 46.9 mm at 2 m (see Table 1)to 49.4 mm at a 186 p. depth of 20 m. That is, the focal length has changed from 35.19 Brgger, S., and A.K. Chong, 1999.An application of close range photog- mm to 37.1 mm based on a waterlair refractive index of 1.3328. rammetry in dolphin studies, Photogrammetric Record, The computed focal length also varies by a small amount from 16(93):503-517. Buddemeier, R.W., and J.E. Maragos, 1974. Radiographic studies of reef coral exoskeletons:Rates and patterns of coral growth, Journal of Experimental Marine Biology and Ecology, 14:179-200. T~LE4. ESTIMATEDAGE OF THE OBSERVEDCOLONIES AT THE EUSAB~ISLAND STUDYSITE Buddemeier, R.W., 1978. Coral growth: retrospective analysis, Coral Reefs: Research Methods (D.R. Stoddart and R.E. Johannes,edi- Colony No. 14 5 7 8 11 14 tors.), UNESCO, London, United Kingdom, pp. 551-571. Total length (mm) 59 107 175 81 139 97 129 Chong, A.K., and K. Schneider, 2001. 'ho-medium photogrammetry Age (year) 27 48 79 36 63 58 58 for bottlenose dolphin studies, Photogmmmetric Engineering 6. Remote Sensing, 67(5):621-628.

750 / ,Iy 1001 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING Clarke, T.A., and J.G. Fryer, 1998. The development of camera calibra- Miller, K.J., 1995. Size Frequency Distribution of Red Comls at Te tion methods and models, Photogrammetric Record, Awaatu Marine Reserve, Doubtful Sound, Fiordland, Report to 16(91):51-66. the Department of Conservation, Invercargill, New Zealand, 14 p. Cubbage, J.C., and J. Calambokides, 1987. Size-class segregation of -, 1998. Population Structure and Growth Rates of Red Coml in bowhead whales discerned through aerial stereo-photogramme- Doubtful Sound, Fiordland, Report to the Department of Conserva- try, Marine Mammal Science, 3(2):179-185. tion, Invercargill, New Zealand, 13 p. Dawson, S.M., C.J. Chessurn, P.J. Hunt, and E. Slooten, 1995. An inex- Peirano, A., C. MOM, and C.N. Bianchi, 1999. 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