for animal tracking data

Derived variables in Env-DATA

Movebank and the Env-DATA system calculate several variables relevant to understanding animal movement, particularly aerial movements, that can be linked to your animal tracking data using the Track Annotation Service.

Topography

The Env-DATA System calculates attributes describing terrain using elevation estimates from three digital elevation models (DEMs): the ETOPO1 Ice Surface Global Relief Model, the SRTM 1 Arc-Second DEM, and the ASTER ASTGTM3 Global 30-m DEM. These attributes are not currently available through Movebank but can be provided upon request. Contact support@movebank.org for assistance.

  • Slope is the change in height divided by the change in distance. Aspect is the compass direction in which the terrain is facing, that is, the downhill direction of the slope. The east-west and north-south components of slope are calculated using four neighboring pixels and describe both slope and aspect of the terrain. These calculations are used as input to orographic uplift calculations described below.
  • Rugosity is a measure of the "roughness" of the land surface, typically calculated as the actual area of the surface divided by the aerial (planimetric) area. Rugosity is provided as the standard deviation of the height (in meters) over a given number of pixels of the original elevation model. Details about how the actual surface area is calculated are in Jenness 2004.

Wind conditions

Wind conditions can greatly affect animals' flight. For example, birds can use upward air movements as an efficient way to travel by soaring and gliding, and can modify their flight timing and behavior based on the wind speed and direction. Orographic and thermal uplift calculated by Env-DATA represent conditions at the resolution of the original datasets and are not intended to replicate the effects of small-scale wind turbulence and changes in topography. Despite this limitation, estimates at the scale provided by Env-DATA have demonstrated correlations between animal movements and wind conditions—see the references below for examples.

Orographic uplift

Orographic uplift happens when rising terrain forces air to move upwards to higher elevations. This is most famously the cause of the "rain shadows" found in mountaineous regions around the world. It is also a consistent source of upward-moving air that migrating birds such as raptors can use to travel long distances along mountain ridges. Estimates of orographic uplift velocity provided by Env-DATA are calculated using estimates of the slope and aspect of the terrain, as described above, and estimates of surface wind speed and direction. For details of the equations used, see Bohrer et al. (2012).

Movebank calculates orographic uplift using four different combinations of source data:

Which version to use will depend in part on the location of your dataset. The NCEP North American Regional Reanalysis (NARR) dataset covers only North America (from about 50–150°W and 12–60°N).

Thermal uplift

Thermals, or buoyant eddies of air, are formed by heating of the air by the sun near the earth's surface. Soaring birds that are slowly circling upward are using this thermal uplift to stay aloft. The estimates of thermal uplift velocity provided by Env-DATA are calculated using estimates of temperature, humidity, surface pressure, boundary layer height, and instantaneous moisture and surface heat fluxes from the ECMWF Global Atmospheric Reanalysis. For details of the equations used, see Bohrer and others (2012).

Other wind conditions

Some additional helpful conditions can be calculated yourself by annotating U and V wind (wind speed in the E-W and N-S directions, respectively) from the NARR or ECMWF weather models.

  • Wind speed and direction can be calculated from U and V wind as nicely illustrated in a course exercise from George Mason University.

  • Wind support and cross wind are measures of the wind conditions in relation to the direction in which an animal is moving.

You can calculate wind support, crosswind and airspeed in MoveApps using the Wind Support and Modelling App, as demonstrated in the Wind Support Evaluation workflow.

References

Bohrer G, Brandes D, Mandel JT, Bildstein KL, Miller TA, Lanzone M, Katzner T, Maisonneuve C, Tremblay JA. 2012. Estimating updraft velocity components over large spatial scales—contrasting migration strategies of golden eagles and turkey vultures. Ecol Letters. 15:96–103. https://doi.org/10.1111/j.1461-0248.2011.01713.x

Jenness JS. 2004. Calculating landscape surface area from digital elevation models. Wildlife Soc Bull. 32(3):829–839. https://www.fs.usda.gov/treesearch/pubs/20437

Katzner TE, Brandes D, Miller T, Lanzone M, Maisonneuve C, Tremblay JA, Mulvihill R, Merovich GT. 2012. Topography drives migratory flight altitude of golden eagles—implications for on-shore wind energy development. J Appl Ecol. 49:1178–1186. https://doi.org/10.1111/j.1365-2664.2012.02185.x

Mandel JT, Bildstein KL, Bohrer G, Winkler DW. 2008. The movement ecology of migration in turkey vultures. P Natl Acad Sci USA. 105(49):19102–19107. https://doi.org/10.1073/pnas.0801789105

Nourani E, Bohrer G, Bierregaard RO, Duriez O, Figuerola J, Gangoso L, Giokas S, Higuchi H, et al. 2021. The interplay of wind and uplift facilitates over-water flight in facultative soaring birds. Proc Roy Soc B. 288(1958):20211603. https://doi.org/10.1098/rspb.2021.1603

Safi K, Kranstauber B, Weinzierl R, Griffin L, Rees EC, Cabot D, Cruz S, Proaño C, Takekawa JY, Newman SH, et al. 2013. Flying with the wind: scale dependency of speed and direction measurements in modelling wind support in avian flight. Movement Ecol. 1:4. https://doi.org/10.1186/2051-3933-1-4

Shamoun-Baranes J, Leshem Y, Yom-Tov Y, Liechti O. 2003. Differential use of thermal convection by soaring birds over central Israel. Condor. 105(2):208–218. https://doi.org/10.1093/condor/105.2.20810.1093/condor/105.2.208

Terms of Use

These derived variables may be used freely for research, conservation, and education purposes. In any publications or presentations that use the data, please cite Movebank and Dodge et al. (2013) (citation below) as well as the original source of the data used to calculate the variable. We also request that you contact us at support@movebank.org to let us how you are using the data.

Dodge S, Bohrer G, Weinzierl R, Davidson SC, Kays R, Douglas D, Cruz S, Han J, Brandes D, Wilkelski M. 2013. The Environmental-Data Automated Track Annotation (Env-DATA) System: linking animal tracks with environmental data. Movement Ecol. 1:3. https://doi.org/10.1186/2051-3933-1-3