Geoscience Reference
In-Depth Information
three catchments areas (represented by green colors) with no outlets. As in
this example,
watershed
ot en generates unrealistic results since watershed
algorithms are sensitive to local minima that act as spurious sinks. We can
detect such sinks in the SRTM data using the function
imregionalmin
. h e
output of this function is a binary image in which pixels containing local
maxima are marked by a value of one and all other pixels are marked by a
zero.
sinks = 1*imregionalmin(SRTM);
h = pcolor(sinks);
colormap(gray)
set(h,'LineStyle','none')
axis equal
title('Sinks')
[r c] = size(sinks);
axis([1 c 1 r])
set(gca,'TickDir','out');
h e pseudocolor plot of the binary image shows twelve local sinks,
represented by white pixels, that are potential locations for spurious areas
of internal drainage and should be borne in mind during any subsequent
computation of hydrological characteristics from the DEM.
Flow accumulation
, also called
specii c catchment area
or
upslope
contributing area
, is dei ned as the number of cells (or area) contributing
runof to a particular cell (Fig. 7.14). In contrast to the local parameters
of slope and aspect, l ow accumulation can only be determined from the
global neighborhood. h e principal operation is to add cell inl ows from
topographically higher neighboring cells, starting from the specii ed cell
and working up to the watersheds. Before adding together the outl ows from
each cell we need to determine the gradient of each individual cell towards
each neighboring cell, indexed by
N
. h e array
N
contains indices for the eight
adjacent cells, according to the MATLAB convention, as shown in Figure
7.13. We make use of the
circshift
function to access the neighboring cells.
For a two-dimensional matrix
Z
, the function
circshift(Z,[r c])
circularly
shit s the values in the matrix
Z
by
r
rows and
c
columns. For example
circshift(Z,[1 1])
will circularly shit
Z
one row down and one column to
the right. h e individual gradients are calculated by
for the eastern, southern, western, and northern neighbors (known as the
rook
'
s case
) and by
