Environmental Engineering Reference
In-Depth Information
2.4.1.2 Creating a Digital Elevation Model
For creating a DEM of the wetland, we recommend using either the “natural
neighbor” method or the “kriging” method. Both methods are commonly used for
creating a DEM, but there are many choices for grid interpolation methods. Given
that we are working with less than thousands of data points, you can create a TIN or
you can create a surface by interpolating points to make a raster. We describe
kriging in this example. We created a synthetic wetland from our data and
sub-sampled it using a variety of sampling grid spacing to synthesize the accuracy
achieved by different levels of effort in surveying (Fig.
2.18
). We then created
both TINs and krig DEMs to illustrate how accurately each one represented the
original synthetic wetland in terms of volume, which was analyzed at a variety
of depths.
Figure
2.18
shows the rate at which accuracy improves as more survey shots are
taken, which results in tighter survey grids being used. However, you should also be
aware that there is much less accuracy at lower water stage compared to higher
stage. From our experience, we found that kriging improved accuracy between
2 and 10 % in the 5 m grid survey and by 11-30 % in the 10 m grid survey. The
results indicate that the advantage of kriging increases markedly when the survey
has fewer shots to control elevations in the Molera Wetland.
We now describe the procedure for kriging. Using the “ArcToolbox” (icon with
a red tool box), select Spatial Analyst
Kriging with a hammer
symbol. Next, you should select the point features created and select the Z value as
“Elev”. Finally, you must define the results into a DEM folder within your project
directory. We used the default options for this kriging. ArcGIS creates a default
output cell size, but we rounded the value to 0.1 m. Changes to the “Maximum
Distance” for kriging “search radius” might improve the output depending on the
bathymetric variability, but we left the value blank (Fig.
2.19
). The resulting DEM
will appear in the project.
Finally, we created a polygon shape file as a mask that can be used to trim the
kriged surface so that we do not extrapolate elevations beyond the GPS collected
points. To do this, we opened the ESRI ArcCatalog program and navigated to our
project via the Folder Connections where you can right click the folder
Interpolation
>
>
>
shapefile and select polygon as file type. We call it “KrigMask,” and assign the
project's coordinate system. You now add the newly created shapefile to the map in
ArcMap and open the editor menu where you select the correct shape file to edit and
then go to construction tool and select polygon. After selecting polygon, you can
create a polygon on the boundary of the wetland points, and use the aerial photo to
help define where to click each point. To end polygon construction, you must
double click. You can now save your edits, and then click “stop editing” to
complete the process. To further refine the DEM, you should select Spatial Analyst
Tools
>
new
Extract by Mask and create a new trimmed DEM (e.g.,
Krig_Trim) using the DEM as a raster and the mask shape file (Fig.
2.20
).
Extraction
>
>
