Elevation inputs

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Elevation inputs

Post by rcoville » Tue Aug 04, 2015 9:46 am

Project elevation information can be input as either: a Topographic Index (TI) file, developed for use in hydrological models; or a Digital Elevation Model (DEM) file, commonly used in GIS software for a variety of purposes.

TI files have been pre-processed and loaded into i-Tree Hydro to make it easier for users to get it up and running without the need for technical skill in GIS. These are available for many watersheds and notable places in the United States.
For watershed areas: users can select from a list of HUC8 scale watersheds by State and name or ID; or users can select TI files for watersheds based on their associated stream gauges (the gauge at the point in a stream which the watershed feeds) for a variety of watershed scales.
For non-watershed areas: users can select by State, County, or Place within i-Tree Hydro Step 1. Place in this case refers to municipalities.

DEM files must be clipped/shaped to represent the project area, should be projected to the appropriate UTM coordinate system, and need to be converted from Raster to ASCII, ultimately saved as a DAT file, for input into i-Tree Hydro. DEM files can be obtained from http://nationalmap.gov among other sources such as local government, and geoprocessing in GIS software is usually required to process DEM files for use in i-Tree Hydro projects. This geoprocessing will likely involve: watershed delineation based on the coordinates of a stream gauge or pour point for watershed areas, or project area delineation for non-watershed areas; projecting GIS data; converting GIS data to other file types for use in i-Tree Canopy or Hydro. For more information about using DEM files in i-Tree Hydro please see the i-Tree Hydro User Manual.

Choosing a DEM file (resolution and source):
Generally i-Tree Hydro is flexible enough to work with a wide variety of DEMs. We do not have limiting criteria about what resolution is right for what project scale, because it is case-by-case depending on project size, terrain variation, and DEM resolution and quality.
There are some suggestions for choosing DEM resolution - the following is paraphrased from one of i-Tree Hydro's model developers, Tom Taggart:
Generally, there are at least three things that need to be considered when choosing DEM resolution:
  1. Initial processing time – A 10 m resolution DEM will take longer to process than a 30 m one. Since a total of 9 10m grid cells will fit in one 30m grid cell, initial processing/modeling of the DEM (the first time it is loaded into a project) will take nine times as long. Subsequent modeling of the project using the same 10m DEM, which only uses a processed output derived from the DEM, takes the same amount of time regardless of the DEM size or resolution.
  2. Quality of the DEM - For this model, we need DEMs that show the bare earth, regardless of whether it's pervious or impervious. So, we want DEMs to be clean of buildings and elevated highways, etc. Most DEMs produced by national level agencies are 'bare earth' DEMs, or at least have been processed to the best of current abilities to produce a 'bare earth' DEM. If producing your own DEM, it is important to consider the best methods for producing a high quality 'bare earth' DEM.
  3. Peculiarities of the Topographic Index concept - At certain resolutions and with certain terrains, TI values could come out as negative numbers or with insufficient variation. This is rare, but possible.
    Some examples of these rare TI issues:
    • With a very fine resolution DEM (e.g. 2 m) that has very steep slopes (e.g. 80+ degrees), TI values can come out as negative numbers. If this occurs the TI file can be corrected by adding the inverse of the minimum value to all TI values (i.e. if the lowest TI value is -2, then +2 must be added to all TI values to correct the file). This may also be resolved using a coarser or finer resolution DEM. Negative TI values will cause problems in correctly modeling the water balance for a project within Hydro and so they must be addressed.
    • With a very coarse resolution DEM and a small project area with little variation in terrain, TI values may not vary enough to populate the TI histogram i-Tree Hydro v5 uses. This can generally be resolved using a finer resolution DEM.
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