A three dimensional model of a particular land feature can be
very useful for visualization. A 3D model which can be manipulated in a digital
environment is even more useful, especially if volume calculations need to be performed.
A good software package for this purpose is, "Surfer". It has a
relatively simple user interface and is easy to learn.
Essentially what Surfer requires to create a 3D surface
model is a set of spot heights along with their northing and easting coordinates.
The data can be imported into Surfer by many ways, of which using a MS Excel
file or an ASCII text file are simplest. The software will interpolate the data
points and create a surface. The algorithm used for interpolation can be
changed according to the requirement.
Obtaining spot heights of the area can be done by performing
a level survey. If high accuracy is not required, a quick and easy method of
obtaining spot heights is to overlay a grid with known coordinates over a topo
sheet with contours. Using the contours, the heights of the vertices of the
grid can be determined. Since the northing and easting of the grid are also
known, it can be fed into Surfer via MS Excel Another method of creating a 3D model using Surfer is to
directly import a DEM (Digital Elevation Model) into the software. DEMs can be
downloaded from the USGS website.
A simple volume calculation based on spot heights obtained
by the gridding method is illustrated below.
First, the grid was drawn over the contour map in the
AutoCAD environment. In this example, the contour interval is 2 m, and the grid spacing has been selected as 3 m.
The spot heights were found by using the contours closest to
the vertices of the grid and the data was entered as x, y and z coordinates
into an MS Excel sheet.
Using Surfer, the excel sheet was input and a grid file was
created. (Grid>Data). The gridding method used for this example is
"Kriging". Other methods can be used depending on the requirement.
A 3D surface was created using the grid file previously
created.
This 3D surface can be used for volume calculations as well.
For example, Consider that the trough indicated in the above terrain model
needs to be filled with water, up to the 8m contour level and it is required to
calculate the volume of water required.
This means we need to calculate the volume between the
modeled surface and the plane z=8. To do this, the volume calculation dialog
box was opened (grid>volume..) and the upper surface was given as z=8 and
for the lower surface, the grid file was selected. The calculated positive
volume indicates the volume of water required to fill the trough. In this example the calculated volume is, 696.73 cubic meters.