Elevation Monitoring Methods
Elevation
The goal of elevation monitoring is to quantify changes in topography that result from restoration, and to relate those changes to responses in hydrology, vegetation and fauna.
Topography and hydrodynamics determine the composition and distribution of tidal marsh habitats and the availability for fish and birds to access them. An important drainage and habitat feature of these marshes are tidal channels, influencing biological and physical characteristics such as plant and avian distributions, fish and invertebrate establishment, and soil biogeochemistry (Callaway et al. 2001, Zedler 2001, Hood 2006, Hood 2007). Repeated measurements of LiDAR, topographic mapping, and bathymetry provide useful information regarding the development of geomorphic features over time. Elevation surveys can be compared to pre-restoration elevations for a spatial analysis of change detection.
Multiple methods are used to measure elevation changes in tidal marshes and should be chosen based on restoration and monitoring objectives and site specific considerations. Here we present information on the following methods for measuring elevation changes over a broad spatial scale. For repeated measures at more localized spots, we recommend referring to methods detailed on the Sediment webpage.
- Ground-based topographic mapping using RTK GPS
Description: Uses real-time-kinematic global positioning system (RTK GPS) to collect high resolution elevation data at the landscape level. Can be used to create digital elevation model (DEM) of a study site.
Benefits: mobile, collects data quickly, measures elevation within an accuracy of 1 – 3 cm, good spatial coverage
Limitations: expensive, can be time consuming when mapping large study sites - Bathymetric mapping
Description: For underwater applications. Transducer is mounted on boat/kayak and measures water depth, RTK GPS measures location. Points are interpolated to produce a bathymetric map.
Benefits: Fast, accurate, lots of data, excellent spatial coverage, can be used to measure vegetation height; with right software can calculate volumetric differences between different time periods
Limitations: expensive, cannot penetrate vegetation or water to measure bare earth surface elevation - Terrestrial LiDAR
Description: LiDAR unit mounted on tripod emits laser pulses. Distance and angle of surface reflectance is recorded and x, y, and z positions are calculated. Produces 3D surface map.
Benefits: Fast, accurate, lots of data, excellent spatial coverage, with right software can calculate volumetric differences from different times
Limitations: expensive, ideal for unvegetated areas above water (i.e. mudflats), smaller sites (several acres to several hundred acres) - Aerial LiDAR
Description: LiDAR unit mounted on airplane for large spatial coverage. Distance and angle of surface reflectance is recorded and x, y, and z positions are calculated. Produces 3D surface map.
Benefits: Fast, accurate, lots of data, excellent spatial coverage
Limitations: expensive, ideal for unvegetated areas above water (i.e. mudflats)
References
Callaway, J. C., G. Sullivan, J. S. Desmond, G. D. Williams, and J.B. Zedler. 2001. Assessment and monitoring. Pp 271-335 In J. B. Zedler, editor. Handbook for Restoring Tidal Wetlands. CRC Press. Boca Raton, Florida.
Hood, W. G. 2006. A conceptual model of depositional, rather erosional, tidal channel development in the rapidly prograding Skagit River Delta (Washington, USA). Earth Surface Processes and Landforms. Online only.
Hood, W. G. 2007. Scaling tidal channel geometry with marsh island area: a tool for habitat restoration, linked to channel formation process. Water Resources Research 43: W03409.
Takekawa, J.Y., I. Woo, N. D. Athern, S. Demers, R. J. Gardiner, W. M. Perry, N. K. Ganju, G. G. Shellenbarger, and D. H. Schoellammer. 2010. Measuring sediment accretion in early tidal marsh restoration. Wetlands Ecology and Management 18(3): 297-305 doi: 10.1007/s 11273-009-9170-6.
Zedler, J. B. 2001. Handbook for Restoring Tidal Wetlands. CRC Press. Boca Raton, FL. 439pp.