L-19 Applications of DEMs in Viewshed and Flood Hazard Mapping


Hello everyone, and welcome to a second last
topic of ah digital elevation models and application course. And in this ah topic, we are going
to discuss ah the view shed analysis, and ah also we will in the last we will be covering
about the how flood hazard mapping can be done using digital elevation models. ah If
you if you recall the previous discussion when we were we have been discussing about
how to use digital elevation models in solar energy estimations, there also we have discussed
about the view shed, but that view shed is a hemi spherically upward looking view shed.
Here ah it would be looking this view shed analysis is based on ah horizon that means,
parallel to the surface of the earth, so that is the major difference.
Of course, there will be some other and differences and options which will which are going to
be available when we go for view shed analysis. And also we will be discussing where view
shed analysis ah can be applied in our engineering applications. ah Also sometimes in the literature
you may find a different term ah which is also called visibility analysis. So, instead
of view shed one can also call visibility analysis. So, what we are going to discuss
here that what is view shed basically, view shed is the identification of areas of terrain
using a digital elevation model that can be seen from a particular point on the surface
of the earth. . So, this this is what view shed is we may
we may ah determine if view shed in a particular direction or maybe a 360 degree we may add
some offsets to the target may be offset of the person who who is going to look from a
point or a height of a tower. These all these things we will be looking. .
So, in this like in this example on the right side what we are seeing this is the location
ah of a ah point from where the view shed analysis was done ah in the terrain. And ah
on if we restrict that only on one side, I want to ah analyze then these green areas
are showing that from this point these green areas would be visible and the remaining areas
will not be visible from that point . Because if you see very carefully you would find that
ah this point is located on the slope which is roughly north facing and therefore, southern
facing slopes and all those sides will not be visible at all . So, this is what the identification
of areas of terrain that can be seen from a particular point on the surface and this
is what the example is . And we also see that ah in view shed ah identifies
ah the cells because it is what we are implying is a digital elevation model which is a raster
base. So, it identifies the cells in an input DM that can be seen from one or more observation
locations . In view shed analysis, you can also involve not only one observation ah location,
but multiple observation locations as well. So, through a point five that information
can be provided along with offsets another thing and then a combined view shed analysis
can also be performed . And the this operation the view shed operation
and both say of course, a digital elevation model which is the main input here or a digital
terrain model or maybe a TIN. So, on TIN also which is ah another terrain surface representation
can also be employed for view shed of operations or analysis . And they say view shed when
we use a DEM to calculate new set of a particular point what we basically require that the location
of that point. So, x y location and then of course, ah what is the height and that you
know so we can add that one. So, in this like a point x can be seen from surface y may be
target location is there and what part of surface y can be seen from point x.
So, if I can see a point y from location x then I should be able to see the same x from
y location . And the multiple point view shed can also be combined to calculate the view
shed of line and areas. ah And where ah they say ah the when when the part of feature x
can be seen from on surface y, ah and and what part of surface y can be seen ah from
which point on the feature x . So, when we involve multiple points then analysis becomes
little ah little complicated, but does not matter . And this can be achieved quite easily
only the interpretation part may be little confusing , when we involve the multiple observation
points . So, while calculating this view shed a line
of sight ah from the observer point to the terrain surface is calculated , and ah which
ah search the inter visibility matrix ok ah through which it calculates the inter visibility
matrix . And visible parts of the terrain surface is an example I have shown earlier
that ah some parts of the terrain where visible from that particular location and in none
way visible areas ah or dead areas can be assigned a different colour .
And ah this ah we can as mentioned earlier that we can add a offset of the terrain offset
from the observation point, because ah if a person is standing then his height can be
added as offset or a person is standing on a table or building top of a building or roof
of a building, then the height of the building can become offset. So, that that offset is
also added . And ah if ah if it is related with some turbine or some other thing that
height can also be added . If we see the example here that what we see
here and that a person is having offset A is ah located here there are persons which
are having offsets ah B. So, ah we can analyze along a this is a of course, a profile section
, but we see here that ah the person A can see ah partly the person standing in this
ah nv area or on the top of the hill that person can fully see. But the person who is
behind this hill cannot be seen by this ah even offset or a person standing at this hillock
ah. So, this is without offset B if we do not
add the offset v, then this is how the analysis nv not visible, here not visible and this
part is not visible . And ah whereas, ah with the with the offset B then we are having having
that upset then entire they up from this location; that means, from this location up to this
location and the entire area is visible . But except this part that is the other side of
the slope which is not visible . And this is of course, an example here with
the offsets ah in a two d or just along a profile, but ah in ah when we imply digital
elevation model we can see all around and it is not only along a profile but all around.
Maybe as per requirements we can go do this analysis for 365 degree as well .
So, what are the parameters which control or ah in the input which are required while
doing the said analysis as mentioned these offset A and B that means, observer . And
target offsets can be there and then we can restrict the azimuth that in only in between
this azimuth and ah like in this example azimuth ah 1 and azimuth 2, I want to analyze this
ah view shed or creative view shed. And then I can also restrict the vertical angle that
how how ah up above from horizon it will look, so that ah horizon is here . So, this vertical
one and vertical two can also be incorporated . And then radius that ah what is going to
be the radius of that ah area whichever one would like to analyze.
So, involving in this these kind of inputs one can determine a view shed as per desired
or as per requirement. So, it is possible to limit the reason of the raster or DEM which
is inspected by specifying various items as or various options as ah given or can be provided
through attribute table. And there for example, observation point ah elevation values, vertical
offsets, and horizontal and vertical scanning angles, and scanning distances how far ah
the analysis is required. May be end of the boundary of digital elevation model or if
you are covering a very large or if you are analyzing a very large digital elevation model
then you can restrict the distance that the scanning distance is only that much required
. And this is the example of view shed analysis
along a line as ah line of sight you can say . And on the same DEM which we have been using
in this course the example DEM . And ah if a person is standing here the line of sight
has been drawn like this then these are the areas which would be in the green areas which
would be visible or part of the terrain . However, in this line of sight there are areas marked
red will which not be visible . And ah of course, this is a hill shade just to bring
the ah ah terrain in relief and relations, so that ah this view shed has been overlaid
ah or rather in background the hillside has been kept . And so here we like in simple
analysis we can provide the observer of shed and that would be in the units which your
elevation model is having four vertical scale. So, this is 5 meter, and the ah target offset
is given 10 meter and then analysis is performed . And of course, in the profile also you can
see the same analysis that ah this is the line of sight visibility profile. So, these
red parts cannot be seen whereas, then green part this is that corresponding green part
is seen . Then again you do not see a small patch of the terrain is here mark red and
then a large part the green part is shown here, then again a red patch and then finally,
a green patch . So, likewise plan view can also be seen in
a line of sight as well as a profile , but ah when you are going for a 360 degree then
ah only in one particular direction or along a line of one particular direction, you can
have ah this profile. Otherwise, you say in only in case of a line of sight this profile
can be determined . If we go for a view shed analysis along a
single point in a circular fashion and ah by giving this radius, what we have restricted
that the distance it has scanning distance it has to cover is ah as per the radius given
here. The point here and the areas which would be visible are all shown in the pink colour,
and rest of the areas are will not be visible from that point . So, likewise ah such a analysis
can be done only along in line or in a radius or depending on.
Now, here the input parameters the offset was again kept a 5 meter, a target offset
was 10, field of vision that means, in it has to look all around that means; that means,
the 360 degree, one can also restrict to say 100 degree or 90 degree or 180 degree depending
on the requirements. And ah less the degree we will keep quickly the analysis will be
done by the system . And near distance how close you want and how far distances it should
cover . So, all that ah can be given as options. And once ah the analysis is done such outputs
can be created . We will see some applications also. So, instead
of having a circular approach ah one can also have a rectangular approach that ah I want
only the visibility analysis in this particular rectangle whose centre is here . So, the observer
is here . And ah then again the red areas from that point ah which is light blue colour
cannot be seen . Whereas, the green areas are only seen. Here also a different tool
in ah rgx and this ah visibility analysis again brings some options here offset A, offset
B, azimuth one, azimuth two, vertical one, vertical two and all these options are available
for our analysis . Similarly, ah view view shed analysis can
also be done for entire digital elevation model inputted digital elevation model . So,
ah we can do the view shed analysis along a line that is line of sight. We can restrict
ah in a particular direction, we can restrict we can restrict and under a circle whose radius
we can decide or a rectangular or even for entire digital elevation model as shown here
and that the observer is here and ah then submitted for entire area. So, red areas are
not visible from this location get that particular offset which was given here .
Whereas, the green areas are visible in this one . So, ah multiple examples can also be
added and ah multiple observer observers can also be added as one can see here that a view
shed has been done for five observers here . So, again as a convention is that ah that
ah view shed analysis has been done, so the low visibility or ah rather no visibility
for the red areas green will build the visible from these points. And the range is there
between 0 to 5 . And this ah this means that ah if a a yellow colour is coming that means,
from ah and roughly from ah two or three locations ah that and that area can be seen or two observation
points. So, that is why there is a range between 0 to 5 . ah
5 means from all five observation points and ah that area that is shown here is is green
colour can be seen. Whereas, complete red means ah from and no other remaining points
means ah four points or and no points that area can be seen . So, there are five observation
points . And suppose this is the area which cannot be seen by any of these observation
points. So, five means ah the ah terrain part of the terrain can be seen from all observation
points. And zero means and from none of the observation points .
And ah if if ah we want to just classify and between visibility and non-visibility instead
of a range like here in this region and is telling us between 0 to 5, you want just a
sort of binary that visible and not visible then this analysis can also be performed like
this . So, only if visible from any point then it will be considered as green; if it
is not visible from all points then it will we considered ah and none points, then it
will be considered not visible . So, not visible part is little less ah compared to whereas,
green area that is visible part is maximum because five observation points and the classification
has been done based on just visible or not visible . So, once the output has been created
or review shed has been created then we can classify in that case when we are involving
more than one observation points . And there are some other examples also there,
there are there that there are two types of visibility analysis that can be performed
here also that ah the two observers are here in this example. We see only the visible part
whereas ah with the same observer locations , but ah we want to see which one are visible
which part of the terrain is visible from observer one, which part or of the terrain
are visible from observer two and which part are visible from ah both the observers . So,
all three possibilities are also there. So, this can be also analyzed like this .
So, this frequency means ah this determines that from how many locations a part of terrain
is visible. So, frequency determines which raster surface locations are visible to a
set of observers . ah If we are having five observers, whether it is visible from five
locations or three, or two or one whatever . And observers it identifies which observers
are visible from each raster surface. So, the analysis has to be done from both ways
as is given in the beginning that a location x should be visible from y and vice versa
is also true because it cannot be one way the visibility will always have the direct
connection . Now, when when we are doing this analysis
for ah for ah terrain or the when large distances are involved, and therefore when visibility
ah analysis being done o over a large area, then two other parameters will play very important
role. And one is the curvature of the earth, and another one is the refraction . And these
corrections then have to be performed if our line of sight is very long, or if our search
area or a scanning area is very large because if it is there then these two parameters will
play very important role. And if if the corrections are not incorporated, then the analysis will
not be very reliable . So, the view shed will generate may not be very reliable .
So, use of earth curvature correction of sun to correct the curvature of the earth and
refraction , and now with the modern GIS softwares these options are becoming now available.
So, we do not we do not have to ah basically program the things, but now only thing we
have to understand that how much correction will be required in so on and so forth . So,
if I take the example of line of sight then observer of set is 1, consider is say in meters
and the target offset ah do not have any and so it is a 0 just surface. And when we apply
for earth curvature then ah these these are the values which ah when ah we opt for that
then these are the values we will come in the default for refraction and earth radius
correction for radio waves and others. And when we go for applying this thing we
get the ah correction performance. So, corrections are made when projection information for surface
is present, and that means, that ah that in the digital elevation model has to be projected
in some map projection. If it is just in degree decimal non projected situation, then such
corrections of earth curvature and refraction cannot be incorporated. So, one has to remember
this thing that ah the corrections are made only when the projection information for surface
is present or the digital elevation model has been projected in some projection system
may be say UTM or poly conic or whatever . And in addition the ground units and surface
z unit must be in feet meters or unit meters that means, both units horizontal unit and
vertical units should be the same , and the formula used for this correction is something
like this that ah z actual that is the height z actual equal to z surface which will come
from digital elevation model minus distance two and the diameter of the earth . And then
plus ah refraction ah factor multiplying by distance two divided by diameter earth.
So, with this ah with this correction, we can ah perform ah more reliable ah view shed
analysis for when a large area or longer very long line of sight is involved . And these
the refraction or curvature of the earth are very important in case of ah targeting some
missiles and other things, because and these missiles travel for very long distances and
therefore, refraction and earth curvature plays very very important role .
So, these ah that when these are that the distances two mean distances are mentioned
here. So, distance the planimetric distance between the observation feature in observed
point diameter of the earth, refraction, this this refraction is the refractive coefficient
of the light . And the default value for diameter of the earth is defined this much meters , but
when it is projected then accordingly it will be taken . So, and this coefficient is 0.13
ah is taken and that is why ah in this here 0.13 has already been taken ah in this to
apply for refraction correction as a default value . .
So, ah by when involving this ah thing our visibility analysis may be little different,
but ah if area is large then only we see much difference is while involving earth curvature
correction and the refractivity ah refraction correction . But in for a small area probably,
we may not see much differences if we even been incorporate these corrections.
So, this is ah visibility or view shed analysis for a terrain area .
Again ah along in line this is example from digital elevation or tin ah triangulated irregular
non-network on which also we can perform the same view shed analysis. This is the example
here that this is the observer location this is the target location which part would be
visible. And ah which are given in green colour which part will not be visible are given in
the red colour . So, ah the input can be either your digital elevation model that is raster
or can tin also Now, ah this is the example how one can perform
in arc GIS. And ah when I choose all these things I have to give ah this is in line of
sight so I am giving a direction and ah and a target location and offset another values.
And once you do it, then you get this kind of analysis. For example, here only a small
part of the terrain here which is visible, rest are not visible. Because if offset has
not been added, much offset has not been added, then there are chances that the ah through
visibility analysis, you would find that only a very small path along the line of sight
is visible and ah same would be in case of if I go for a rectangular or circular area.
So, offset has to be added generally some offset at least human height has to be added
if one is targeting ah from the top of a building or a tower observation tower then and that
offset has to be added . Now, how how other these corrections ah play
very important role as we can see that ah here. And let us assume that an observer O
is ah located at sea level that is looking towards the target here that AT ah located
again at the sea level . And now the earth curvature how it is playing important role
. So, for the calculation, we will assume the earth is is a sphere , but ah you know
that for perfect calculations ah few more parameters have to be incorporated ah because
earth is not a perfect is sphere, nonetheless ah less for here we are assuming that earth
is a perfect sphere. Then lets ah let us get the radius R that is ah the earth of the earth
at the observer O and at the target location here also at AT.
Now, the radius of the at the target will intersect the tangent at observer at point
T here . So, ah this ah lesson this indicates the sync of the target and instead of here
it is coming here due to curvature of the earth as S. So, this S factor has to be incorporated
So, using Pythagorean theorem, we can easily obtain the value of the sync how much drop
is there and that is R plus S square equal to R square and D square. And ah likewise
we can calculate so this can be solved for s as a quadratic equation. And ah then we
get the perfect ah the value of the S which is D 2 D square by ah 2 R plus S .
So, ah for different distances and this has this think how much sync effects will come
or drop effects will come has been calculated for 1 kilometre, 5 kilometre, 10 kilometre,
ah 30,000 kilometre, 50,000 kilometre, these these number of ah meters and the sync would
be there. So, ah it plays very important role ah for the sync . And ah only this ah correction
or such corrections are required when large distances are involved our calculations in
view shed analysis ah . Also a little bit about how atmospheric refraction
ah will play important role as ah we can see here that ah these atmospheric ah different
atmospheric layers which surrounds the earth. We are having the position of the sun actual
position of the sun , but because of refraction effect we get the apparent position of the
sun somewhere here . Same way our observations or in view shed analysis, instead of getting
the actual position, we get the ah the target position apparent position something like
this. So, atmospheric refraction is the deviation of light or other electromagnetic wave from
a straight line as it passes through the atmosphere due to variation in air density as a function
of a height. So, when it passes through different layers, it bends and the position is somewhere
else . So, taking into account the refraction of
the light as a general atmospheric condition, the density of air decreases at height increases;
and as a result of this the lights tend to bend as it travels long distances through
air . And this causes distance objects near the horizon to appear higher than they are
actually are . And instead of feeling here and you make it you may have a feeling that
ah that the target or ah and the height of that location or point is much above . And
this negates ah to some extent the sinking caused by the curvature of the earth , but
not fully, so that the refraction coefficient might differ for different atmospheric conditions,
but ah since ah we do not have ah regular availability of such values. So, ah 0.13 can
be taken in most of the cases. So, this ah refraction coefficient this is how it is derived
. Now, there are examples of ah view shed analysis
or line of sight analysis ah with the different ah sinks are there. And ah that ah compared
with the profile of the surface with increase of the distance to the observer . So, how
things are ah changing here as you can ah see very well here that observer ah is here.
And ah then what we see the changes ah in the visibility part of that ah as one can
see very easily here . So, ah if we change ah this ah the distances and the sinks will
affect , and therefore, we will have a different view shed analysis.
Here in the first example, and there is no difference and whereas, when we ah add a few
more things then the differences ah becomes large as you can see. So, the green part here
of the surface will be visible ah along this line of sight profile and the red parts are
not visible . And the effects of earths curvature can be seen here . And this is the line of
sight of radio propagation , and this is the ah effect of atmospheric refraction. So, these
ah factors curvature and refraction probe and ah changing atmospheric conditions may
bring some changes as depicted here; otherwise ah we may get analysis something like this.
So, actually that analysis should not have been like this for view shed this should ah
have been like this in this one . Now what are the applications of visibility
analysis . ah There are various applications ah when ah mainly for observations or locations
one can use that ah for visual impact analysis. Somebody is going to construct ah a tower
or building or something and before that they would like to see that how much part of a
valley or hill would be visible from top of ah that building or from the window of different
floors, they they can analyze. Also when somebody is constructing ah these observation towers
maybe in a ah forest area or maybe in a mining area or for security purposes or for telecommunication
then ah this view shed analysis can play very important role. ah These mobile operators
nowadays are using ah view shed analysis to see the level of signals which would be available
in the target locations. Also we can involve in landscape evaluation
that ah how things are visible, and ah they can be organized. And ah this also as I have
just mentioned the sighting of observation towers where the which is going to be the
best location for such towers of cellular communication must or towers that can be done.
ah And we can model these coverage of cellular communications or mainly mobile communications.
Of course, military applications are there I have given the example and related with
missiles and other things, where these ah corrections or errors have to been ah taken
care especially curvature of the earth and refraction and ah changing atmospheric conditions
if that much data is available. Then live analysis can also be done . And
ah virtual GIS ah also where this flight simulators and other things are done where also in those
analysis the visibility analysis is going to be very very important. This virtual GIS
was ah developed on war footings when we had this Kargil war. And a digital elevation models
were employed and ah top of that then latest satellite images were trapped and then pilot
were trained before they were dispatched for the site, ah so that ah this virtual gis can
help and also in that one this view shed in all these things . And this is not a exhaustive
list. So, there can be many more applications of visibility or view shed analysis [noise
And this is one of the examples of ah how view shed analysis in ah on Mars it has been
done, because human cannot reach. So, when ah somebody or some equipment is being sent,
so we one would like to assess what would be the view shed or for that equipment . So,
this Mars exploration rover MER project and this ah USGS used a view shed analysis ah
ah to assist this ah Mars explorer that when it lands on the surface of the Mars which
area would be visible which will not be visible and how ah it will be ah doing the exploration
part. So, that ah that was done in advance. Because digital elevation model of mars was
available, so view shed analysis could be done .
And when ah this we want to find out ah what NASA wanted to find out the appropriate landing
spots for mars rovers they turned to the USGS map for the best possible sites basically
the USGS maps of ah Mars . And part of analysis included a view shed of possible site selections
and in this case the view shed indicates the areas which may may not be visible for Mars
rovers from each landing site . So, different sites were evaluated and the most appropriate
ah site was finally, then selected for landing of this Mars explorer rover .
Now, in the last ah very briefly I will touch how digital elevation model can be used for
flood hazard monitoring planning and other things. As you know that ah digital elevation
model is representing the undulations of a terrain . So, along a floodplain ah of river
ah that they can also be utilized it can be simulated and that if water level in a river
goes to this level then how much area would be inundated , this kind of simulation I have
ah we have discussed when I was discussing dam simulation . So, almost the same tools
can be used along ah river valley ah or across a flood plain. And we can ah instead of assuming
dam, we can assume a water level and then ah we can simulate or model that how flooding
would occur if water level in a river goes like this .
So, this is the one of that example here and given that this is purely DEM based ah flood
monitoring or flood planning rather ah that ah this geomorphological processes that are
directive in floodplain areas are as you know that related with the terrain especially fluvial
geomorphology I am talking. And the DEMs are the most suitable data for such kind of analysis.
And we can perform not only the qualitative, but quantitative analysis that where exactly
ah the area which areas would be inundated how much water would be there and so on and
so forth . And ah historical elevation data if are available
then sedimentation and erosion rates may also be derived from digital representation of
relief over a space. So, if a if a flood has happened in past, we want to estimate that
how much ah you know ah sediments fluvial sediments it has created, how much erosion
was there what was the rate probably these things can also be calculated. But for which
probably we may be requiring several successive ah digital elevation models maybe in time
series . So, ah this longitudinal profiles can also
be drawn ah and cross profiles can also be drawn any plan views of a through a digital
elevation model of an underrated area can also be drawn. So, this DEMs can
really help not only for the view shed analysis of different types as we have discussed, but
also related with the ah flood hazard monitoring planning and ah you know similar kind of studies
where terrain data is involved. So, this brings to the end of this discussion.
Thank you very much. .

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