The instructions below describe a generic set of steps to convert the outputs of a hydraulic model and prepare them for use as inputs in the habitat model software.
An Important Note About Rasters!
The habitat software involves overlaying multiple rasters and extracting values from all the rasters at precisely the same location. This demands that the rasters for each input are perfectly aligned. i.e. the cells for each input layer line up on the Earth in precisely the same location. This GIS concept is called orthogonality. We also require that all inputs have precisely the same overall spatial extent, which is called concurrency.
It can be tricky to ensure orthogonality and concurrency; especially in modern GIS packages like ArcGIS that allow the overlay of layers that do not share these properties and instead will automatically resample the underlying rasters without the user’s knowledge. However, orthogonality and concurrency are easier to controll for if all the rasters are divisible. Divisibility means that the corners of each raster cell (and therefore the entire raster extent) are evenly divisible by the cell resolution. So if a raster has a cell resolution of 1m then the corners of the raster must end in whole metres.
The 1m resolution depth raster in the image below is non-divisible. It has an irregular extent that is not evenly divisble by 1 and it would be extremely hard to produce any other raster that perfectly aligns with this one.
In ArcGIS it’s important to identify the step in your workflow when first creating any raster and ensuring that you set the processing extent settings to ensure that the output raster is divisible. Avoid taking an existing non-divisible raster and simply resampling it to be divisible as this will alter the actual cell values.
Professor Joe Wheaton at Utah State University has several excellent lectures on this subject if you need more information.
Raster Requirements for Habitat Model Inputs
- GeoTIFF raster format (*.tif file extension).
- Divisible and concurrent rasters.
Steps to Produce Rasters From Hydraulic Model Results
The following steps assume you have the ArcGIS desktop GIS software package and the 3D Analyst extension license.
Manual Process
- Create a point ShapeFile of the computational nodes with the output variable (usually velocity or depth) as a column attribute.
- Obtain or draw a channel perimeter polygon ShapeFile around the outer extent of the area that you want to model habitat.
- Create a TIN surface using:
- the point ShapeFile has mass points with the input variable as the Z value.
- the polygon ShapeFile as a hard clip input.
- Use the TIN to Raster tool to convert the TIN to a raster making sure that you:
- open the environment settings and set the processing extent to divisible coordinates.
- append the .tif file extension to the end of the raster name.
- Review the outer extent and cell size of the resultant raster and ensure that the raster is divisible.
Python Script
James Hensleigh of Utah State University (USU) has written a Python script that automates the steps described above. The same ArcGIS requirements apply.