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Benthic Habitat Mapping
Temporal scale and time constraints are important to consider when planning a benthic habitat mapping project. If the area under study is subject to seasonal changes or fluctuations due to particular events, this will affect the timing of data collection. It may be desirable to collect data during a certain time of year or during several different times of year. Seasonality Studies that involve mapping submerged aquatic vegetation (SAV) using aerial photography should be based on imagery acquired during the season of maximum biomass or flowering of the dominant species. This will often occur when most plants or communities are at their peak. This peak is June for SAV of the Pacific Northwest and Atlantic Northeast, April and May for the southeastern U.S., and September in the eastern U.S. While biomass may be high during the summer months, in many areas the strong potential for runoff from convective storms and haze during this season limits the ability to acquire adequate photography. The images below show the apparent differences in seagrass coverage due to the timing of data collection. The image on the left was acquired in the spring while manatee grass was in full foliage. The image on the right was acquired later in the season when above-ground biomass had died back. However, there is no real loss of seagrass between the two dates.
For more details on mapping SAV using aerial photography, please refer to Guidance for Benthic Habitat Mapping: An Aerial Photographic Approach (PDF) or Submerged Aquatic Vegetation: Data Development and Applied Uses CD-ROM. Tidal Conditions Timing of data collection around tidal stage may be an important consideration. Boat access and maneuverability may be limited in shallow areas of a study site around low tide. This would limit sampling times and likely affect the overall data collection period. Tidal stage can also be an important consideration in collecting airborne imagery. For example, the State of South Carolina is currently collecting digital multispectral imagery to map intertidal oyster beds. To maximize mapping efficiency and obtain very high-quality imagery, the data must be collected around very low tides, which occur only a few times per month. Since image quality is also affected by sun glare, the flights can only be conducted at times of the day when the sun is greater than 45 degrees above the horizon. As a result, image acquisition is constrained to a few hours, on a few days, when low tide occurs with a sun angle greater than 45 degrees above the horizon. Although this is an extreme example, it demonstrates how time constraints can factor into project planning. Sun Angle Whenever optical imagery is collected over water, the problem of sun glint must be addressed. The objective in optical mapping is to collect imagery when the most light is reaching the bottom and thus potentially returning to the sensor. Unfortunately, the potential for glint increases along with bottom illumination, especially at high sun angles (as measured from the horizon). How much effect glint will have on an image is based on sun angle, sea state, the focal length of a sensor, and its field of view. In general, glint is most problematic when sun angles are high and the observing sensor has a wide field of view. Collecting imagery only during certain times of day can minimize sun glint. Sun angles above 30 degrees are sufficient to provide useable bottom illumination; however, as sun angles exceed 45 degrees, the possibility of glint problems arises. Collecting stereo imagery (images with overlap) can also reduce the effects of glint in a data set. In this way, at least one image will be free from glint in the affected area. Wind Effects A calm sea state is essential to successful benthic mapping using airborne optical sensors. Foam from breaking waves will obscure submerged features and smaller waves will also reflect light, which hinders the ability to image the bottom. Airborne missions can be timed so as to minimize the effects of wind. Usually this involves collecting imagery in the earlier part of the day before winds pick up due to convective heating. Missions can also be scheduled to avoid the period immediately after strong winds. This will reduce the amount of swell on the surface. Avoiding recent wind events will also minimize the possibility that bottom sediments are resuspended by wind, thus creating turbidity that hampers the imaging of bottom features. A general guide for minimizing wind effects on optical remote sensing is to collect imagery only when winds are below five miles per hour and ocean swells are below three feet. In addition to the effect that wind has on optical remote sensing, winds and swell can hamper collection of acoustic, videography, and other sampling data from small boats. In these instances, winds may blow a boat off a target site before a measurement can be made. This can be a more serious problem in cases where an observation at a very specific location is desired or where the water is deep and the boat may drift significantly before the instrument reaches the bottom. For acoustic data collections, high sea conditions due to winds cause the angle of the sound wave to vary greatly, as it is pulsed out and received. These angle anomalies must be resolved to correctly geolocate the data. |
