Accuracy Assessment of Wetland Boundary Delineation Using Aerial Photography and

Digital Orthophotography

Jeffrey Barrette, Peter August, and Francis Golet

Abstract derived from the same aerial photo (Heric et al., 1996). The qual- We compared the horizontal accuracy of forested wetland ity of the orthophotography for wetland delineation is often boundary delineations obtained from large-scale, color dependent upon the spatial resolution (or pixel size) of the aerial photography against delineations obtained from "heads- image. This is mostly a function of scale (Manzer, 1995); the up" digitizing of digital orthophotography The absolute value larger the scale, the smaller the pixel size, the clearer the image, of the mean (+- so) distance between the field-derived "truen and, therefore, the easier it is to identify wetland boundaries wetland boundary location and the o~h~photograph-derived (Federal Geographic Data Committee, 1992; wilen and smith, wetland boundaries was 3.4 ? 3.4 m and was more 1996). Although the horizontal accuracy of digital orthophotog- accurate than the mean distance between the "truen boundary W h y is far superior to aerial photographyl Nale (1995) argues and the aerial-photo-derived wetland boundaries (4.5 +- 4.0 that photo quality (i.e., sharpness and clarity) is more important

m). ~ i ~ l d visitation increased the accuracy of delineating than horizontal accuracy. There have not been critical, quanti- wetland boundaries, and the majority of wetland edges-both tative comparisons of the positional accuracy of wetland o r ~ ~ o p ~ o t o g r a p ~ - and photo-derived-were on the boundaries determined using traditional aerial photography wetland side relative to the 'true' boundary. and orthophotography.

This study compared the horizontal accuracy of wetland Introduction delineations obtained from aerial photo interpretation against Conventional aerial photographs are an essential source of data wetland delineations obtained from digital o r t h o ~ h o t o g r a ~ h ~ . for natural resource scientists; however, geometric distortion We followed standard methods of aerial photo interpretation due to camera tilt, relief displacement, lens distortion, and (Tiner, 1990)~ recompilation, and digitization (Bolstad et a]., atmospheric refraction result in scale variation throughout an lggO) using photography* and used ''heads- image (Burnside, 1985; Alberts, 1992; Bolstad, 1992). This dis- UP" (on the screen) digitizing of digitalortho~hotog- tortion produces horizontal error when attempting to register r a ~ h ~ (Logan, lgg3; Lillesand and Kieffer, 1994). Using global photo-interpreted data into a geographic information system positioning system (GPS) and GIS tools, we measured the dis- (GIS) (Bolstad et al., 1990). In contrast, orthophotography is tances each the derived sets scale-accurate across the image (Smith, 1995). boundaries to the "true" wetland edge based on field reconnais-

~ i ~ i ~ ~ l o r t ~ o p ~ o t o g r a p ~ s are produced by scanning an sance at each of the established assessment sites. aerial photo diapositive, orthorectifying the digital image, and registering it to a coordinate system and map projection ( ~ o o d , Methods 1989; Novak, 1992; Manzer, 1995; Hohle, 1996). The resulting Study Area orthophotograph is free from scale, tilt, and relief distortions We obtained digital o r t ~ o p ~ o t o g r a p ~ y for a 15-km~ area located that are characteristic of conventional aerial photography. The in the City of Warwick, Rhode Island (Figure Theodore

Orthorectif~ing~ and geographic registration steps are Francis Green Memorial State Airport is located just north of technically complex and require sophisticated equipment (Lil- the study area and Greenwich Bay defines the majority of its lesand and ~ i e f f e r , 1994; smith* 1995); thus* o r t h o ~ h o t o g ~ a ~ h ~ southern boundary. The area consists of 77 percent developed can be very expensive to produce Manzerl lgg5). lands (e.g., residential, commercial, and industrial); 18 percent

photography has been the natural lands (e.g., forests, brushland, open water, and wet- used source for and (Tiner lands); and 5 percent miscellaneous (e.g., farms and beaches). and Smith, 1992; Tiner, 1996bl and other natural resources. ~ i ~ h ~ ~ - ~ ~ ~ hectares offreshwater wetlands exist in the project One advantage of using aerial photos for wetland delineation area, with the majority consisting of deciduous forested wet- is that aerial photo pairs can be viewed stereosco~icall~l Per- lands (38 ha) and open water (32 ha). The remaining wetlands mitting 3D visualization of terrain variation (Dickinson, 1979; consist of emergent wetland and scrub-shrub classes. Glacial Wolf, 1983) and identification of wetland boundaries (Tin% outwash covers 95 percent ofthe land area, till covers percent, lggO). Another advantage is that photography some- and open water covers 3 percent (cover estimates taken from times provides a sharper image than the orthophotograph

J. Barrette is with Environmental Systems Research Institute, Inc. - Olympia, Suite 300, 606 Columbia Street NW, Olympia, Photogrammetric Engineering & Remote Sensing

WA 98501-1099 (jbarrette@esri.com). Vol. 66, No. 4, April 2000, pp. 409-416.

P. August and F. Golet are with the Department of Natural 0099-1112/00/6504-409$3.00/0

Resource Science, University of Rhode Island, Kingston, RI O 2000 American Society for Photogrammetry 02881 (Pete@edc.uri.edu; Natres@uriacc.uri.edu). and Remote Sensing

PHOTOGRAMMETRIC ENGINEERING 81 REMOTE SENSING Apr1l2000 4 0 9

Rhode Island

Figure 1. Location of study area within the City of Warwick, Rhode Island, and its forested and non-forested wetlands.

Rhode Island Geographic Information System database [August et al., 19951). Elevations, based on a 0.5-m contour coverage provided by EarthData International of Maryland (Gaithers- burg, Maryland), range from 0 m at the Greenwich Bay shore- line to 56 m near the southwest corner of the project area. The majority of the area (90 percent) is below 15 m.

Data Sources The orthophotography was developed from aerial photographs taken on 28 April 1996 using natural color film (Kodak Aeroco- lor film 2455) exposed at an altitude of 1.1 km above mean ter- rain. The aircraft had a WILD RC30 aerial camera system equipped with forward motion compensation and a 6-inch focal length lens cone calibrated at 153.469 mm (Photo Sci- ence, Inc., 1997). The scale of the photography was 1:7,200. There was a total of 80 exposures covering the study area with 70 percent endlap and 30 percent sidelap between adjacent frames, which was sufficient for stereoscopic viewing (Burn- side, 1985; Smith, 1995).

Although false-color infrared photography is the standard for wetland mapping (Tiner and Smith, 1992; Hershey and Befort, 1995), natural color photography was the only film available for this season that provided a clear view through the forest canopy. Color infrared photographs were taken approxi- mately 3 weeks later, but at a time when red maple (Acer rubrum) foliage had already developed enough to prevent a

clear view to the ground; thus, it was difficult to identify satu- rated soils and understory vegetation (Tiner, 1990).

Digital orthophotography was created by scanning 15 natu- ral color diapositives. The raster scan files were produced with an aperture of 20 micrometers using a Zeiss SCAI flatbed scan- ner (Photo Science, Inc., 1997). The scanned digital images were enlarged by a factor of six, producing imagery at a scale of 1:1,200. The imagery was orthorectified to remove geometric distortions. Fifteen digital ortho quads (DOQS) were produced for this study. Each DOQ was one square kilometer in geo- graphic extent with a picture element (pixel) resolution of 15.2 cm (6 in). Based on the film type and resolution of the imagery, features such as parking stripes, manhole covers, and fire hydrants could easily be identified.

The imagery was projected into the State Plane Coordinate System with units in feet, using the North American Datum of 1983. All of the photogrammetric processing was done by EarthData International of Maryland (formerly called Photo Science, Inc., Gaithersberg, Maryland) and is described in their technical report for the project (Photo Science, Inc., 1997).

Two GPS systems were used during the project. A GPS sur- veying consultant (GPS Services, Inc., Rockville, Maryland) was contracted to establish ground control throughout the proj- ect area with Ashtech SCAlZGPs receivers (Ashtech Inc., Sun- nyvale, California) capable of centimeter-level accuracy. We considered locations derived by this equipment to represent "truth." These control points were used to assess the absolute accuracy of each of the data sources in the study. A second GPS system (Trimble Pro XL, Trimble Navigation, Sunnyvale, Cali- fornia), capable of sub-meter accuracy, was used for locating wetland boundaries identified in the field.

Horizontal Accuracy Assessment Thirty-six clearly defined control locations visible in both the aerial photography and orthophotography were used to assess the positional accuracy of each of the datasets used in the study. Clearly defined locations included grassy corners where con- crete sidewalks met asphalt driveways at 90°, the center of man- hole covers, and crossed parking stripes. Four of the Ashtec GPS receivers were used for the assessment; one served as a base station and three as rover units to survey control point loca- tions. Each receiver was set to track five or more satellites for 30 minutes, and positional fixes were logged every 10 seconds. The locations of control points had clear to moderately ob- structed horizons, and no problems were encountered during the survey or post-processing steps. The positional fixes for each point were averaged into a single location and were hori- zontally accurate to within 5 cm of the actual location on the ground (Collins, 1996).

Geographic coordinates (in units of state-plane feet) for the 36 control points were obtained using digital orthophotography, aerial photography, and the sub-meter accurate Trimble Pro XL GPS system. Coordinates were obtained from the digital ortho- photography by "heads-up" digitizing while viewing the digi- tal orthophotography as a backdrop in ArcView 3.0 (Environ- mental Systems Research Institute, Redlands, California). The coordinates for each of the control points on the aerial photo- graphs were obtained by registering the photos onto a Calcomp 9100 (Calcomp Inc., Anaheim, California) digitizing tablet con- nected to aData General 5220 workstation (Data General Corp., Westboro, Massachusetts) using ArcIInfo rev 7.0.4 software (Environmental Systems Research Institute, Redlands, Califor- nia). Clearly defined points on the aerial photograph were used as registration marks (tics). For each tic, easting (X) and north- ing (Y) values were derived from the digital orthophotography. All photographs were registered using four tics, and each con- trol location was digitized as a point feature. Coordinates defin- ing control points were also measured using the Trimble Pro XL GPS system by taking 25 position fixes at 5-second intervals

410 April 2000 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING

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at each location. The 25 position fixes were differentially cor- boundaries. Sites were initially selected using aerial photo- rected and averaged into a single location (August et al., 1994; graphs and were placed at least 30 m apart and under forested Deckert and Bolstad, 1996). canopy.

The mean easting (X-value) and northing (Y-value) coordi- The location of each wetland boundary site was measured nates for the resulting point data sets were compared against using the Trimble ProXL (sub-meter accuracy) GPS receiver. At the "true" Xand Y coordinates derived from the sub-centimeter each location, 25 position fixes were recorded, differentially accurate (Ashtec) GPS receiver. Tests of the equality of means corrected, and averaged to a single position fix. Wenty-five for all three sources were performed using an analysis of vari- replicate fixes were chosen because significant improvements ance (ANOVA) (Sokal and Rohlf, 1981). The means of each point in accuracy are realized with this level of averaging (August et data set were also evaluated using an independent t-test (Sokal al., 1994). The appropriate settings (Trimble, 1994) for sub- and Rohlf, 1981) against the expected horizontal accuracy for meter accuracy were used. All GPS data were collected during each data source based on manufacturer's specifications of the leaf-off conditions to maximize positional accuracy under a for- equipment (e.g., nimble ProXL = 0.9 m) or National Map ested canopy (Wilkie, 1989; Gerlach and Jasumback, 1989). To Accuracy Standards of the source data (e.g., digital orthopho- decrease the required time to lock into a minimum of four satel- tography = 1.0 m, aerial photography = 6.1 m). Statistica soft- lites (Deckert and Bolstad, 1996) and to minimize distortion ware (Statsoft Inc., Tulsa, Oklahoma) was used for all caused by multipathing (Hurn, 1989; Hurn, 1993), we mounted statistical tests. the GPS antenna onto a telescopic pole that could reach 8.7 m

above ground level which extended it well into the forest can- opy (approximately 12 to 16 m high). The pole was used to ele-

Wetland Delineation vate the GPS antenna for all forested canopy sites. The GPS- The freshwater studied were based UPon derived coordinates for wetland boundaries were considered accessibility, owner's permission, and no variation in wetland1 the "truev location for all analyses. upland edge type. Due to the small number and low variety of The two wetland boundary data sets (i.e., derived from wetland types throughout the project area, there were not orthophotography and aerial photography) were compared to enough sites to evaluate the effects of different edge types on the positions of the 128 boundary sites identified in boundary identification; therefore, only forested wetlandlfor- the field. Distance from the "trueM point location was measured ested upland transitions were assessed. This type of wetland to each data set. This was accomplished manually by overlay- edge was selected for two reasons: (1) it is the most common in ing point data sets on the boundmy data sets on the the project areal and (2) it is one of themore difficult edge types computer screen, zooming in as close as possible, and measur- to delineate (Tiner, 1990; Tiner and Smith, 1992; Wilen and ing the distances with the tool in A ~ ~ v ~ ~ ~ . lfthe Smith, 1996). wetland delineation was located upland relative to the "true"

Forested wetlandlforested upland boundary transitions position, a positive distance was recorded. If the wetland were initially identified using color aerial photography. Ap- delineation was located inside the wetland relative to the proximately 50 percent of these areas were field-visited during "true" boundary, a negative distance was recorded. the wetland delineation phase of the study. The remaining Because the raw data were, in many cases, not normally areas were not visited until all delineations were complete. We distributed, nonparametric statistical tests were used (Sokal split wetland delineations into field-visited and non-field-vis- and Rohlf, 1981). The absolute values of distance from each ited categories to test if field visitation improved the overall delineation type to "truth" were compared using a Man-Whit- accuracy of wetland delineation. Field-visited sites were ney U test to evaluate which method was more accurate. A Wil- selected arbitrarily. coxon matched pairs test was used on the raw data values to

Aerial photographs were used to navigate to the field-vis- determine if statistical differences existed between mean devi- ited wetland areas. In the field, the approximate wetland ations fcom truth. If the mean was not equal to zero, a direc- boundaries were identified (Environmental Laboratory, 1987) tional bias existed; a wetland-side bias if the mean was nega- and marked onto an acetate overlay atop the photo. Forested tive; and an upland-side bias if the mean was positive. A wetland boundaries were "heads-up" digitized while the Mann-Whitney U test was used to determine if the accuracy of orthophotography was used as a backdrop on the computer boundaries for field-visited sites was the same as the accuracy screen. Aerial photographs were used to facilitate the "heads- for unvisited sites. The sample size for field-visited sites using ,

up" digitizing process, but were never viewed with a stereo- aerial photography was very low (N = 5); therefore, we only scope during this phase. Boundaries were created and saved used data derived from the orthophotography for the analyses in the same coordinate system and datum as the digital of the effects of field verification. orthophotography.

The same forested wetland boundaries described above E M of Forest Canopy on GPS Accuracy were delineated again using the 1:7200-scale color aerial pho- During an attempt to improve the precision and accuracy of the tographs, this time using a stereoscope. Stereo photo pairs and GPS locations measured at each wetland site, we discovered a 3X mirror stereoscope (Topcon Instrument Corporation of that the forested canopy significantly altered the GPS positions. America, Paramus, New Jersey) were used to delineate the wet- When processing GPS data using Trimble Pathfinder Office soft- landlupland boundary onto an acetate overlay with a liquid ware (Trimble Navigation, Sunnyvale, California), the user is ink pen equipped with an 0.18-mm tip. Field-checked wet- able to associate standard deviations with each GPS location, lands were revisited when necessary to make the appropriate providing that multiple position fixes are averaged into a single adjustments to the photo-interpreted delineations. Acetate fix. We revisited 35 wetland sites with standard deviations > sheets with wetland delineations were registered to the digitiz- 0.6 m. When we compared the newly obtained locations to the ing tablet and digitized using the same methods established original locations for these 36 sites, we noticed that the differ- during the horizontal accuracy assessment. ences between the two were as high as 12.2 m. This suggested

A total of 128 marked (59 in visited areas and 69 in non- that multipathing within the forested canopy was taking place. visited areas) wetland boundary sites were established This effect was independently tested at two separate locations; throughout the project area. "True" wetland boundaries were one within our study area-approximately 30 km from the base identified at each site using U.S. Army Corps ofEngineers wet- station-and another on the University of Rhode Island (URI) land delineation guidelines (Environmental Laboratory, 1987) campus in South Kingstown, Rhode Island-less than 1 km and were represented as point locations along the wetland from the base station.

PHOTOQRAMMETRIC ENQINEERINQ & REMOTE SENSINQ April 2000 411

Although manufacturer specifications indicated that a GPS

svstem *onld provide sub-meter accuracy under leafless for-

eit canopies, we made a number of measurements to verify the

oositionll accuracv of the system. A total of ten test sites were

used for the analysis, five sites at the uRl location and five sites

within the study area in Warwick, Rhode Island. At each loca-

tion (uRI, Warwick), two sites were located in open sky condi-

tions and three sites were located under a forested canopy. All

ten sites were visited a total of five times over a 1-week period'

At each visit we collected a total of six points. The first point

was an average of 25 position fixes, and the other five points

were an average of five position fixes. AII sites were measuredusing the same Trimble ProXL cPS receiver we used to survey

the oiiginal wetland sites. The telescopic pole was used duringthese tests.

Because we did not know the true location of each of thetest sites at uRI and Warwick, we measured the average location

of all fixes obtained for each site and computed the deviation ofeach fix ftom this average. We assumed that the average ofthese fixes closely approximated the true location. However'our assessment was actually an evaluation of the "precision"

of coordinates obtained under the experimental conditionsrather than "accuracy. "

A three-way univariate ANovA (Sokal and Rolf, 1981) wasused to evaluate the mean distances against three factors: site(Warwick vs. URI), canopy type (open sky vs, forested), andnumber ofaveraged position fixes per point feature (5 vs. 25)'The Tukey Honestly Significant Difference (Hso) test was usedto test for equality among means.

ResultsHorizontal Accuracy AssessmentThe positional accuracy of coordinates measured with digitalorthophotography, a sub-meter accurate GPS, and aerial pho-tography differed significantly (asov.q,, F : 1o2.92, p < 0.0001;Figure 2). The mean (* 1 standard deviation, n = 36) distancefrom true was o.27 -r o.22 m for digital orthophotography, o.77+ 0.57 m for sub-meter cps, and 3.87 + 1.90 m for aerial pho-tography. Positional accuracy of the digital orthophotographyand sub-meter GPS did not differ (HSDtest, p > 0,05);however,both were significantly more accurate than aerial photography(rtsn test, p < 0.0001). These deviations equaled or exceededthe expected horizontal accuracy for each data source basedupon manufacturer's specifications (for cPS) or National MapAccuracy Standards (Nvas, for aerial and orthophotography).The digital orthophotography was within 0.9 m of true (t: 0.93,p > 0.05), the sub-meter cPS was within 0.9 m (f : 1.46, p >0.05), and aerial photography was much more accurate thanthe 6.1 m expected by NMAS (t : 7.05, p < 0.0001).

Accuracy of Wetland BoundailesThe mean absolute distance from the "true" wetland location tothe orthophoto-derived wetland boundaries (3.e9 * 3.45 m)was significantly less than the mean distance from "true" to theaerial-photo-derived wetland boundaries (+.s3 * 4.05 m)(Mann-WhitneyUtest; Z = -2.53,p < 0.05). We considereddeviations into the wetland as negative distance from true anddeviations into the upland as positive distance from true (Fig-ure 3). The mean directional bias for orthophotography(-1.56 + 4.62 m) and aerial photography (-1.13 + 5.98 m) didnot differ from each other (Wilcoxon matched pairs test; Z :0.83, p > 0.05), but both differed significantly from 0 (Wilcoxonmatched pairs test; orthophotography, Z: 4.O2, p < 0.0001;aerial photography, Z: 1.84, p : O.O7). This indicated a direc-tional bias toward the wetland side of the "true" location forboth sets of wetland-derived boundaries (Table 1).

Field visitation affected the overall accuracy ofwetlanddelineation. Mean absolute deviations from "true" wetland

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X - Deviation (mete6)

Sub-moter Accurate GPS

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X - oeviation (meteF)

Figure 2. X and Y deviations (m) from "truth" for each ofthe three data sources.

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Figure 3. The range of distances (m) between orthophotog-raphy- and aerial photographyderived wetland delineationsand the "true" wetland boundarv.

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4L2 April 2000 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING

TABLE 1. FREQUENCY OF WETLAND OR UPLAND BIAS FOR WETLAND BOUNDARY TABLE 2. MEAN (k 1 SD, in m) DEVIATIONS FROM TRUTH^" BMEEN LOCATIONS DERIVED FROM DIGITAL ORTHOPHOTOGRAPHY AND AERIAL HABITATS (FOREST-CLOSED CANOPY; AND OPEN-NO CANOPY) AND

PHOTOGRAPHY REPLICATE FIELD SITES (UNIVERSITY OF RHODE ISLAND [URI] AND WARWICK, RHODE ISLAND. LOCATIONS MEASURED IN FORESTED CONDITIONS AT BOTH

Aerial Photogmphy SITES WERE MORE VARIABLE THAN LOCATIONS MEASURED IN THE OPEN SITES

Wetland Upland (P < 0.001, TUKEY'S HONESTLY SIGNIFICANT DIFFERENCE TEST). Orthophotography Side Side Total

Habitat Wetland Side 5 1 32 Upland Side 22 2 3

83 Site 45

Forest Open

Total 73 55 128 U R I 2.41 IT 1.76 0.47 t 0.24 (n = 90) [n = 60)

Warwick 1.68 IT 1.30 0.54 2 0.29 (n = 90) [n = 60)

boundaries for orthophotography were 2.97 +- 3.54 m for vis- ited sites and 3.83 3.34 m for non-visited wetlands, and these 2"'hth" Was estimated b y taking the mean of a l l X and Y fixes at each differed significantly (Mann-Whitney test; = 2.22, < field site and habitat condition. The values shown here are the mean

0.05). deviation o f each replicate h o m the average o f a l l fixes.

On average, wetland boundaries derived from the ortho- photography were 4.84 t 4.75 m away from the boundaries assessed using the aerial photography. Only three wetland site be expected under forested canopy conditions (Deckert and boundaries as determined by each method were more than 15 Bolstad, 1996). m away from one another and 75 percent of the boundaries The mean horizontal accuracy determined for the aerial were within 6 m (Figure 4). photography represents a "best-case" scenario. The photo-

graphs were geographically registered using coordinates Effects of Forested Canopy on GPS Accuracy derived from the orthophotography. Another base map of com- The sub-meter GPS receiver performed significantly better (t = parable or better scale than the aerial photography was unavail- 10.49, p < 0.0001) under open sky conditions (average devia- able for recompilation and digitization of the photo-inter- tion of 0.51 t 0.27 m) than under forest canopies (average devi- preted wetland delineations (Bolstad, 1992). The only other ation of 2.05 + 1.58 m; Table 2, Figure 5). Although there was no difference between the two test areas (URI, Wanvick) under open sky conditions, differences did exist under forested con- ditions. The total number of position fixes averaged together to estimate a location had no effect on the overall accuracy in open sky conditions (t = 1.84, p > 0.05) or forested canopy con- ditions (t = 0.40, p > 0.05).

Discussion The assessment of the horizontal accuracy of the three data sources used in this study was performed to validate the expec- ted accuracy for each data source based on N U S or manufac- turer specifications. Digital orthophotography, aerial photo- graphy, and sub-meter accurate GPS all met or exceeded the expected positional accuracies. However, the tests of the accu- racy of GPS were done in open sky conditions. It became obvi- ous later in the study that similar levels of accuracy should not

60

50

t u 40

I 30

CI Po z

10

0

O ~ * ' ' ' . ' ' ' ' ' " N I O " R Z & X : :

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Figure 4. Frequency plot of distances between wetland edges derived from orthophotography and from aerial photographs.

Open Sky Conditions

12

8 - VI u

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-12

X - Deviation (meters)

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x - Deviation (meters)

Figure 5. Distribution of averaged GPS points relative to the mean center location for open sky and forested canopy conditions.

PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING Apri l 2000 413

alternative to this method would be to use GPS to obtain coordi- nates for at least four registration marks per photograph and to use those coordinates to register the photography into the com- puter (Kruczynski and Jasumback, 1993). We used the coordi- nates derived from the orthophotography to geographically register the aerial photographs for two reasons: (1) it saved time by avoiding the process of identifying registration marks on the aerial photography, visiting the sites, collecting GPS positions, and post-processing the data; and (2) the mean distance from "true" between the orthophotography and GPS-derived control points was not statistically different.

Wetland delineations obtained from digital orthophotogra- phy were more accurate than delineations derived from the aerial photography. The resolution and clarity of the orthopho- tography were sufficient to easily identify freshwater forested wetlands (Mead, 1981; Scarpace et al., 1981; Logan, 1993). The resolution of the imagery used in this study was exceptional for the purpose of wetland de1ineation.l

Wetland boundaries derived from aerial photos were, on average, approximately 1.07 m farther away from "truth" than the same delineations derived from the orthophotography. This difference is approximately equal to the pen width (0.02 cm on the photo, 1.3 m on the ground) used to delineate the wetland boundaries on the aerial photography and is well within the expected horizontal accuracy standards for 1:7,200-scale pho- tography (6.1 m). The statistical difference between the two boundaries is likely due to the difference in scale between the orthophotography and aerial photography.

The similarity in accuracy of the two methods of wetland boundary delineation may be explained by the nature of the wetlands in the study area. In most cases, the wetlands were sit- uated in low-lying basins with obligate wetland plant species (e.g., Woodwardia virginica and Symplocarpus foetidus) (Tiner, 1996a) growing to the edge of the wetland. The edges of the wetlands usually abutted a steep slope, and the transition from wetland soils to upland soils (Environmental Laboratory, 1987) was usually no more than 3 to 5 m. The darker toned wetland areas contrasted sharply against the lighter colored uplands forming a clearly definable edge (Tiner, 1990).

Viewing the aerial photography in stereo did not provide significant advantages over viewing the digital orthophotogra- phy in 2D. The positional accuracy of wetland delineations derived from the ZD orthophotography was 1.07 m better than the same delineations made in 3D using stereoscopic pairs of aerial photos.

We attempted to evaluate the effectiveness of superimpos- ing (e.g., draping) the digital orthophotography onto a 3D sur- face created from the DTM used to produce the orthos (Cibula and Nyquist, 1987; Garbrecht and Starks, 1995) to visualize changes in slope near the wetland edge. The computer pro- cessing time to draw each view was more than 8 hours. To use the method effectively, each wetland needed to be viewed from multiple angles and elevations. It was obvious that this proce- dure was not practical using the software and hardware config- uration we had available at that time. However, draping ortho- photography atop a 3D rendering of the landscape would prob- ably be a very useful way to view the imagery if this could be done rapidly. Future enhancements of GIS software and hard- ware will likely make this possible.

Natural color digital orthophotography at this scale and resolution (1:1200,15.2-cm pixel size) is expensive and re- quires considerable disk storage capacity (Toth, 1996). The imagery we used cost approximately $3000/square km to cre- ate, including the cost of the aerial photography, GPS control,

*Samples of the orthophotography have been placed in a world wide web page and can be obtained from www.edc.uri.edu/warwick.

creation of ~ D T M , and photogrammetry. A single DOQ (1.0 square kilometer) of true color imagery is 130 mb in size. The digital orthophotography we used in this study represents the largest practical scale for wetland delineation over a reasonably wide area. At this time, it is unlikely that larger scale and higher res- olution orthophotography could be economically provided for regional (e.g., state, county, or watershed) applications.

Field visitation improved the overall accuracy of orthophoto-derived wetland delineations by an average of less than 0.91 m. We expected field visitation to have a larger effect on horizontal accuracy, as the literature indicates (Tiner, 1996a). This small deviation is likely due to the exceptionally clear and high-contrast signatures of wetland boundaries on the orthophotography and the abrupt wetlandlupland bound- aries in the field. Had we chosen wetlands with a more gradual topographic transition to upland, field visitation might have had a more significant effect. We did not attempt to classify wetland vegetation types during the study but we believe that both field visitation and stereoscopic viewing would signifi- cantly enhance classification accuracy (Tiner, 1996a). Other studies have found that automated classification using digital imagery is usually not as successful as photointerpretation (Mead, 1981; Scarpace et al., 1981; Duhaime et al., 1997; Tiner, 1996b).

For both conventional aerial photography and orthopho- tography, wetlands were more often delineated on the wetland side of truth as opposed to the upland side. This was antici- pated because, when wetlands are delineated using remote sensing techniques, the boundaries are usually delineated dow- nslope of the "true" location (Tiner, 1996a; Golet, personal observation). Usually the interpreter can easily identify very poorly drained soils (i.e., soils that are saturated to the surface during the growing season) and associated dense understory vegetation because these areas appear darker and contrast well with the surrounding drier areas (Tiner, 1996a). The "true" boundary, as assessed by the Federal method (Environmental Laboratory, 1987), usually occurs on the upland side of poorly drained soils, which do not appear as dark and are often inter- preted as an upland area.

Mean distance between the wetland boundaries derived from aerial photography and digital orthophotography (irre- spective of truth) was 4.84 m. This distance is also within the expected positional accuracy of the aerial photography and sug- gests that the difference in the accuracy of wetland boundary delineation is due to scale. At only three wetland sites were distances greater than 15 m; for 75 percent of the sites the dis- tances were within 6 m.

Although the positional accuracy of wetland delineations was similar between the two sources of data, the process of wetland delineation using digital orthophotography was much easier and more time efficient (Logan, 1993). Time differences were not quantified during this study, but we estimate that the entire delineation process was approximately twice as fast using digital orthophotography. Clearly, the time differences would have been greater if GPS-based registration points were collected for each aerial photo or if delineations were recom- piled onto another basemap before digitizing-which is the usual case.

There were several other advantages of using digital ortho- photography over conventional aerial photography in the wet- land delineation process. The ability to use computer software to magnify areas where the wetland boundary appeared to be unclear at particular scales helped significantly. Corrections to the wetland delineations were easier with the digital bound- aries as opposed to ink-on-acetate delineations. Software was used to adjust the red-green-blue values that represent the natu- ral color imagery, and this enhanced the brightness and contrast of the imagery, The ability to adjust the appearance of the ortho-photo image was especially useful in identifying the wet-

424 April 2000 PHOTOQRAMMETRIC ENQINEERINQ & REMOTE SENSING

land boundary in dark areas. "Heads-up" digitizing of wetland Dickinson, G.C., 1979. Maps and Air Photographs, Second Edition, boundaries into an established coordinate system removed the John Wiley & Sons, Inc., New York, 348 p. need for "edge-matching" (DeMers, 1997) delineations that Duhaime, R.J., P.V. August, and W.R. Wright, 1997. Automated vegeta- spanned multiple photographs. Edge-matching wetland tion mapping using digital orthophotography, Photogrammetric boundaries taken from aerial photographs required a consider- Engineering b Remote Sensing, 63(11):1295-1302. able amount of time. Environmental Laboratory, 1987. Corps of Engineers Wetland Delinea-

Despite the effect that forest canopy had on the overall tion Manual, Tech. Rep. Y-87-1, U.S. Army Engineer Waterways horizontal accuracy of the sub-meter GPS, the mean error of Experiment Station, Vicksburg, Mississippi, 100 p. + Appendices. 2.05 m was well within the error associated with identifying the Federal Geographic Data Committee, 1992. Application of Satellite "true" wetland boundary in the field. Considering the range of Data for Mapping and Monitoring Wetlands - Fact Finding Report, deviations (0.21 to 11.76 m) obtained from GPS under forest can- FGDC Tech. Rep. 1, Wetlands Subcommittee, Washington, D.C., opies, the level of confidence in a coordinate obtained for any 32 p. plus Appendices. one site should be low. This error is most likely due to the Garbrecht, J., and P. Starks, 1995. Note on the use of USGS level 1 7.5- effects of multipathing, i.e., the reflection of GPS signals off minute DEM coverages for landscape drainage analyses, Photo- reflective surfaces between the satellite and the receiver (Lang- gmmmetric Engineering 6. Remote Sensing, 61(5):519-522. ley, 1997; Weill, 1997). In this study, reflection of GPS signals Gerlach, EL., and A. Jasumback, 1989. Global Positioning System Can- off tree branches might have degraded the quality of the signal. opy Effects Study, USDA Forest Service Technology and Develop-

This study demonstrates that using digital orthophotogra- ment Program MTDC 89-34, 18 p. phy for wetland delineation was at least as accurate and effi- Heric, M., C. Lucas, and C. Devine, 1996. The open skies treaty: Qualita- cient as using the aerial photography from which it was gener- tive utility evaluations of aircraft reconnaissance and commercial ated. The 15.2-cm pixel resolution was sufficient to clearly and satellite imagery, Photogrammetric Engineering 6. Remote Sens- accurately identify the boundaries of forested wetlands. The ing, 62(3):279-284. process of wetland delineation using the orthophotography Hershey, R.R., and W.A. Befort, 1995. Aerial Photo Guide to New was easier and faster than the process that is traditionally used England Forest Cover m e s , General Technical Report NE-195, for aerial photography (DeMers, 1997). USDA Forest Service, Northeastern Forest Experiment Station,

Radnor, Pennsylvania, 70 p.

Acknowledgments Hahle, J., 1996. Experiences with the production of digital orthophotos, We are grateful to Matthew Nicholson, Keith Killingbeck, Roger Photogrammetric Engineering 6 Remote Sensing, LeBrun, and three anonymous reviewers for their careful 62(10):1189-1194. reviews of the manuscript. Dennis Myshrall and Mary Hutchin- Hood, J., 1989. Image processing techniques for digital orthophotoquad son provided excellent technical support throughout the proj- production, Photog~ammetric Engineering 6 Remote Sensing, ect. Matthew Nicholson, Eric Helms, Charles LaBash, Roland 55(9):1323-1329. Duhaime, Aimee Mandeville, and Margaret Pilaro provided Hum, J., 1989. GPS: A Guide to the Next Utility, nimble Navigation critical field or analytical assistance. Jonathon Stevens was Ltd., Sunnyvale, California, 76 p. instrumental in securing funds for the project. This study was - , 1993. Differential GPS Explained, Trimble Navigation Ltd., supported by the AquaFund Program, administered by the Sunnyvale, California, 55 p. a o d e Island Department of Environmental Management, and Kruczynski, L.R., and A. Jasumback, 1993. Forestry management appli- the University of Rhode Island Department of Natural Re- cations: Forest Service experiences with GPS, Journal of For- sources Science. This is contribution number 3659 of the estry, 91(8):20-24. %ode Island Agricultural Experiment Station. Langley, R.B., 1997. The GPS Budget, GPS World, 8(3):51-56.

Lillesand, T.M., and R.W. Kieffer, 1994. Remote Sensing and Image References Interpretation, Third Edition, John Wiley & Sons, Inc., New York,

750 p. Alberts, D.H., 1992. Distortion: A problem in single aerial photography

GIs World, 5:50-51. Logan, B.J., 1993. Digital orthophotography bolsters GIs base for wet-

August, P., J. Michaud, C. LaBash, and C. Smith, 1994. GPS for environ- lands project, GIs World, 6(6):58-60.

mental applications: Accuracy and precision of locational data, Manzer, G., 1995. Maximizing digital orthophoto use: A technical over- Photogrammetric Engineering 6 Remote Sensing, 60:41-45. view, GIS World, 8(12):50-51.

August, P.V., A.J. McCann, and C.L. LaBash, 1995. Geographic Infoma- Mead, R.A., 1981. Mapping wetlands using orthophotoquads and 35- tion Systems in Rhode Island, Natural Resources Facts, No. 95- mm aerial photographs, Photogrammetric Engineering 6 Remote 1. University of Rhode Island, Kingston Rhode Island, 12 p. Sensing, 47(5):649-652.

Bolstad, P.V., 1992. Geometric errors in natural resources GIs data: Tilt Nale, D.K., 1995. How accurate is digital orthophotography? GIS and terrain effects in aerial photographs, Forest Science, World, 8(12):50-55. 38~367-380. Novak, K., 1992. Rectification of digital imagery, Photogrammetric

Bolstad, P.V., P. Gessler, and T.M. Lillesand. 1990. Positional uncer- Engineering b Remote Sensing, 58(3):339-344. tainty in manually digitized map data, International Journal of Geographic Information Systems, 4:399-412. Photo Science Inc., 1997. City of Wakck , Rhode Island, Final Report,

Gaithersburg, Maryland, 18 p. Burnside, C.D., 1985. Mapping from Aerial Photographs, Second Edi-

tion, John Wiley & Sons, Inc., New York, 348 p. Scarpace, EL., B.K. Quirk, R.W. Kieffer, and S.L. Wynn, 1981. Wetland Cibula, W.G., and M.0. Nyquist, 1987. Use of topographic and from digitized P ~ ~ ~ ~ ~ ~ ~ P ~ Y , Photogmmmetric

logical models in a geographical data base to improve Landsat Engineering 6. Remote Sensing, 55(9):829-838. MSS classification for Olympic National Park, Photogrammetric Smith, G.S., 1995. Digital orthophotography and GIs, Environmental Engineering 6 Remote Sensing, 53(1):67-75. Systems Research Institute Conference Proceedings, Palm

Collins, J., 1996. GPS Report to the City of Warwick, GPS Services Inc., Springs, California, (http://www.esri.com/library/userconf/ Rockville, Maryland, 1 2 p. proc95/tol50/pl24.html)

Deckert, C., and P.V. Bolstad, 1996. Forest canopy, terrain, and distance Sokal, R.R., and F.J. Rohlf, 1981. B i o m e t ~ Second Edition, W.H. Free- effects on global positioning system point accuracy, Photogram- man and Company, New York, 859 p. metric Engineering 6 Remote Sensing, 62(3):317-321. Tiner, R.W., 1990. Use of high-altitude aerial photography for invento-

DeMers, M.N., 1997. Fundamentals of Geographic Information Sys- rying forested wetlands in the United States, Forest Ecology and terns, John Wiley & Sons, Inc., New York, 486 p. Management, 33/34:593-604.

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- i996a. Practical for wetland identification and Trimble, 1994. Datalogging with TDCl Asset Surveyor, nimble Naviga- boundary delineation, Wetlands Environmental Gradients tion Limited, Sunnyvale, California, 281 p. + Appendices. Boundaries, and Buffers (G. Mulamoottil, B.G. Warner, and E.A. Weill, L.R., 1997. Conquering multipath: The GPS accuracy battle, GPS McBean, editors), CRC Press, Inc., pp. 113-137. World, 8(4):59-66. - 1996b. Wetlands, Manual of Photographic Interpretation, Set- Wilen, B.O., and G.S. Smith, 1996. Assessment of remote sensing/GIS

ond Edition (W. Philipson, editor), American Society for Photog- technologies to improve national wetlands inventory maps, Pro- rammetry and Remote Sensing, Bethesda, Maryland, pp. 475-494 ceedings; Sixth Biennial Forest Service Remote Sensing Applica- plus color figures. tions Conference, 29 April -03 May 1996 Denver, Colorado.

Tiner, R.W., and G.S. Smith, 1992. Comparisons of Four Scales of Color Wilkie, D.S., 1989. Performance of a backpack GPS in a tropical rain Infrared Photography for Wetland Mapping in Maryiand, Region forest, Photogrammetric Engineering 6. Remote Sensing, 5, National Wetlands Inventory Report R5-92/03, U.S. Fish and 55(12):1747-1749. wildlife service, Newton Corner, Massachusetts, 15 p. plus wolf, p . ~ . , 1983. Elements of photogrammetry, Second Edition,

Toth, C.K., 1996. Image compression in photogrammetric practice: An McGraw-Hill, New York, 628 p. overview, Digital Photogrammetry: An Addendum to the Manual of Photogrammetry (C. Greve, editor), American Society for Pho- (Received 15 October 1998; revised and accepted 27 March 1999; togrammetry and Remote Sensing. Bethesda, Maryland. revised 07 May 1999)

In October 2000, the American Society for Photogrammetry and Remote Sensing will devote its issue of Photogrammetric Engineering and Re- mote Sensing (PEbRS) to Remote Sensing and Decision Support Systems (DSS). DSS would include the science-based predictive models, remote sensing information, verification and validation, and the communities that conduct the decision support. Authors are encouraged to submit manuscripts addressing remote sensing and GIs contributions to opera- tional Decision Support Systems.

Possible categories of manuscripts include Decision Support Systems

Precision Agriculture (Food and Fiber) Coastal Ecosystem Management (Natural Resources) Flood Plain Risk Assessment (Disaster Management) Water Quality (Environmental Quality) Urban Planning (Urban and Infrastructure) Public Health (Human Health and Safety)

We also encourage the submission of short manuscripts that present the experience of remote sensing1GIS by Federal, State, or Local agencies using decision support systems operationally. Private sector companies under contract to these agencies or otherwise involved in some aspect of developing or operating decision support systems using remote sensing1 GIs technology are also invited to submit a short manuscript.

Guest Editors Ronald J. Birk, Intermap Technologies Inc. Dr. Timothy W. Foresman, University of Maryland-Baltimore

All manuscripts must be prepared according to the "Instructions to Au- thors" published in each issue of PE&RS. Papers will be peer-reviewed in accordance with established ASPRS policy. Please send manuscripts to:

MAY 15 , 2 0 0 0

416 April 2000 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING

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