Modification to the Line of Sight model to accommodate Vegetation

To accommodate the new information layers a new visibility model is required. The model projects the vegetated zones found between the observer and target onto the FOI, so that the façade area visible may be adjusted according to the coverage index, giving an estimate of the area of visible regions on the FOI (Figure 4.8). The FOI is considered as a set of target columns at the resolution of the raster DSM, in this case 1 metre resolution. The visibility is calculated for each target column determining the area clearly visible, and that behind vegetation as a function of the coverage index. As an example if the coverage index for a ray at the FOI is 50% and vegetation covers half of a target column, then the final area visible for that column would be 75% of the column height (50% of 50% in vegetation zone + 50% under and above the canopy).

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Figure 4.8: Projection of Vegetation on to the FOI Façade

The method to calculate the vegetation impact is as follows (supporting Figure 4.9):

a) The angle to the steepest non-vegetated DSM cell is projected on to the FOI to find the Not Seen (NS) zone.

b) The steepest gradient to a vegetated cell between the observer and target is determined, and the lowest canopy angle. This defines the Above Canopy (AC) and Under Canopy (UC) zones. The rays for these regions are subjected to an atmospheric rate of decrease. The NS zone always takes precedence when the other zones are defined.

c) The region on the FOI between the base of the AC zone and the top the UC zone is defined as the Through Canopy (TC) zone.

d) The vegetated regions along the path between observer and target are recorded (e.g. A,B,C) and the intercepts at the FOI are calculated for the top (lines At,Bt,Ct) and base (lines Ab,Bb,Cb) of each vegetated region, defining the TC subzones. Vegetated regions whose projection falls completely within the NS zone are removed, and any base lines which fall in the NS zone are redefined using the angle to the top of the NS zone.

74 e) The vegetated zones are considered in order, furthest from the observer first. To simplify the process an average ray path is used to describe the impact of each vegetated region in each TC subzone (e.g. lines Ax and Ay in detailed view). The permeability values from each cell are applied to the ray to calculate its coverage index, by multiplying the current coverage index by the permeability value, such that a ray experiences an exponential decay. The formula may be written as Coverage Index = 100(p1)(p2)...(pn), where p is the decimal permeability rate in the range 0 to 1 which

determines the likelihood of the ray passing through that cell (see Figure 4.6 for more details).

f) The TC zones are assigned an appropriate coverage index value which corresponds to the presence of vegetation along the LoS. For example the zone between Ct and Cb is assigned the coverage index calculated from ray Cx, the region between Bt and Bb is assigned by Bx, while Ax and Ay are used to assign the remaining A zones. Any regions not assigned a value are known as Inter-Canopy (IC) zones and treated as visible through clear atmosphere (same as AC and UC).

g) The coverage indexes (Ax,Ay,Bx,Cx) are applied to the column zone areas and summed to give a single façade area visible value for the TC zone.

h) Any rays which encounter trunk zones will be terminated.

i) Any rays which have a coverage index of below 1% are terminated and the zone is considered as not visible. As rays fade exponentially this is useful for improving performance for rays which are tending towards 0%.

j) The process is repeated for all target columns within an FOI to give a single visible façade area. A perceived area is also calculated which considers the observer‟s viewing angle and distance (see Section 4.3.2). For interior target cells within a FOI (i.e. not on the FOI boundary) a large proportion of the zoning will be NS, as the majority of the column is blocked by exterior cells.

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Method CalculateVisibleArea_IncVegetation()

# Calculate the Zones on the Target which are:- # Above Canopy - AC

# Through Canopy - TC # Under Canopy - UC # Not Seen - NS TarZ = Target_DSM

#Calculate the Not Seen (NS) Region based on Close Horizon A1= FindSteepestAngle (Non-Veg Cell)

NS = ProjecttoTarget (A1) IF (NS > TarZ)

RETURN 0 #target not visible so return to main routine

#Calculate the Above Canopy (AC) Region based on Vegetation Tops A2 = FindSteepestAngle (Veg Cell)

TCTop = ProjecttoTarget (A2) IF (TCTop < TarZ)

AC = TarZ - TCTop ELSE

AC = 0

#Calculate the Through Canopy Zone Sections - use the LOS from Observer to Target #noting each vegetation block en route from these calculate the intercepts with the target FOR EACH VegBlock in LOS (Observer, Target)

Vt = FindSteepestAngle(VegBlock) Vb = FindLowestCanopy (VegBlock) TCList.Add(ProjecttoTarget (Vt)) TCList.Add(ProjecttoTarget (Vb)) END FOR LOOP

#Sort TC Zones by Elevation TCListSorted = TCList.Sort()

#Calculate the Avg Ray Coverage Per TC Zone

FOR EACH InterceptZ in TCListSorted(0,TCListSorted.Length-1)

IF InterceptZ > NS

#GET MidPt of Each TC Zone

InterceptZ_next = TCListSorted(TCListSorted.Length+1) ZoneHeight = (InterceptZ_next - InterceptZ)

MidPt = ZoneHeight / 2

#calc ray coverage along path to target at designated MidPtZ height (raycasting) #if coverage below 1% then stop search and return value of 0

RayCov=CalcRayCoverage(MidPtZ, 1) RayCoverageList.Add(RayCov,ZoneHeight) END IF

END FOR LOOP #Total Target Area Visible FacadeAreaVisible = AC #TC, UC, IC zones

FOR EACH Zone in RayCoverageList FacadeAreaVisible += RayCov * ZoneHeight END FOR LOOP

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Figure 4.9: Line of Sight Model – projected vegetation zones (A,B,C) onto FOI

To describe the vegetation impact across the FOI façade a new visibility metric is introduced, which divides the FOI into thirds. The zone definitions are created without considering the visibility of the structure, but instead use the extreme FOI target points which create the widest angle from the current location. This is so the visibility results, which are summarised according to these zones, may include the percentages for area visible, visibility through vegetation, and hidden from view, as shown in Figure 4.10. The output quantifies the extent and pattern to which an FOI is blocked from the current view and may be used by LBSs to build natural egocentric descriptions, such as “the gallery is visible in front of you, although partially hidden by vegetation on the right side”.

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Figure 4.10: Summarising Visibility across an FOI in Thirds by Viewing Angle

The next section demonstrates the use of the modified visibility model in a vegetated urban area through a number of trials which illustrate how the adaptations improve the modelled results, and are able to accommodate seasonal variation, views under bridges, and also a demonstration of how the new metric may be used to describe a scene.