3.3 Data Preparation
3.3.3 Altitudinal Belts and Management Compartments
The historic prescribed fire data for the UDP_WHS were obtained from the Ezemvelo KZN Wildlife for a 10 year p eriod (2001 – 2010). T here w ere variations amongst t he y ears in number and position of management compartments with some compartments being overlapped, dupl icated, combined, and /or s ub-divided within and bet ween t he y ears. To develop a study area template, the management compartments were correctly digitised by displaying the y ears simultaneously t o vi ew c ommon occu rrences and v ariations. Where there w ere discrepancies between t he y ears a co mmon do minator w as sort by visually comparing all the years usually resulting in only one of the years being incorrect (Fig 3.3).
There were similar problems with variations in the labelling of the compartments, however there was also common labelling amongst the compartments that were automatically assigned as compartment codes. The code assigned to each compartment was dependent on the previously developed template. Each year needed t o be co rrected and assi gned the appropriate co de t o co mply with t he base t emplate to det ermine t he pr escribed bur ning characteristics of each compartment for each year. Some compartments had two different codes, meaning they would be considered twice in any analysis, which was corrected.
The number of compartments varied between the years (Table 3.3), with the template total being 489 management compartments. Therefore each year had to be corrected for: number of compartments, compartment code and prescribed burning characteristics (burnt/not burnt, type of burn, etc.); i.e. if a co mpartment was required to be sub-divided (Fig 3.3c) then the burning ch aracteristics data of t hat large co mpartment determined the data of t he smaller sub-divided compartments (Fig 3.3a). The original EKZNW digitising process was incorrect resulting in overlaps of neighbouring compartments. This was only realised further along in the process when it became problematic during statistical analysis and these problems were corrected for and discussed in the following section.
Table 3.3: Number of compartments into which the park was divided in different years Year Number of Compartments
2001 586
2002 554
2003 585
2004 603
2005 574
2006 560
2007 606
2008 424
2009 614
2010 536
61 Figure 3.3: Variations in m anagement co mpartment boundar ies between di fferent y ears.
Inconsistencies in m anagement co mpartment b oundaries between t he y ears (dark grey) were corrected using common dominators found within the other years (2005 – 07, 09).
Prescribed burning is carried out per management compartment, (i.e. managers burn by compartment). However these compartments do not coincide with the natural contours and consequently, t he al titudinal bel ts of t he P ark. This is pr oblematic for management as a certain pe rcentage per altitudinal bel t i s required t o be bur nt per year. T here are t hree altitudinal bel ts, n amely m ontane, su b-alpine a nd al pine w hich were det ermined usi ng contour line dat a and b elt r anges (m.a.s.l.). When bur ning a co mpartment t hat falls within two or m ore bel ts, t he total ar ea o f that co mpartment needs to be s ub-divided into i ts respective al titudinal bel ts as a whole co mpartment ca n be pr escribed t o bur n not a su b- division thereof.
The Union tool in ArcMap was utilised to compute a geometric intersection between datasets from multiple sources into a single output feature class preserving the attribute data from all the overlapping input features. This tool was preferred over the Merge tool, which is similar in function, however the Merge tool will not planarize (combine) feature geometries from the input feature classes. However, for both the Union and Merge tool the input data sets have to be o f the same type (i.e. multiple point feature classes, multiple tables, but line feature classes cannot be merged with a polygon feature class), with the Union input features being limited to only polygon geometry.
The input datasets are the altitudinal belts and management compartments and the output feature class is a base map containing the three altitudinal belts (montane, sub-alpine and alpine). T he i ssue bei ng t hat t he al titudinal bel ts are a pol yline f eature w hile t he management compartments have a polygon geometry (Fig 3.5a). Therefore the belts have to be converted to a pol ygon feature. The alpine and montane belts were converted using the
2005, 2006, 2009 2007 2008
a b c
62 downloaded ET Geowizards tool (Fig. 3.4). Closed polylines were created of the two belts (Fig. 3.5b). The Geowizard only converts one closed polyline feature at a time. The reason for hav ing more t han o ne pol yline per al titudinal bel t i s because the a lpine and m ontane belts are intersected by the sub-alpine belt, which is continuous (Fig. 3.5c). When creating closed polylines it is vital that the snapping and finish sketch functions are utilised as they will not be converted to polygons if the polylines are not closed.
These i ndividual pol ygons are m erged and dissolved into t heir r espective altitudinal bel ts, resulting in two feature classes (montane and alpine) (Fig. 3.5d). The two belts were clipped to the boundary of the UDP_WHS (study site) (Fig. 3.5e). The sub-alpine belt was created using t he Erase tool, r emoving t he montane and al pine belt f rom the st udy si te, t he remaining section was the sub-alpine belt (Fig. 3.5f; Fig. 3.4). These three altitudinal belts are use d individually an d t hey ar e m erged t ogether t o create one belt base template (Fig 3.5g). T he union tool was then r un usi ng the al titudinal bel ts and m anagement compartments, both with polygon geometries.
Figure 3.4: The development of the altitudinal belt polygon feature class (consisting of the alpine, sub-alpine and montane belts) from polyline feature classes using the ET Geowizard Tool.
Due t o t he co mpartments not co inciding with t he al titudinal bel ts, t he c ompartments were sub-divided after the union ( Fig 3 .6). The sub-divided co mpartments hav e t he sa me compartment co de as the par ent co mpartment how ever t hey di ffer i n su rface a rea and altitudinal belt into which they belong. The total area burnt for each altitudinal belt is required and therefore when burning the compartment (e.g. fig 3.6a) which falls in all three belts (e.g.
Fig 3.6b), the total area has to be sub-divided and added to the total area burnt of each belt.
Montane Altitudinal Belt
(Polyline)
Alpine Altitudinal Belt
(Polyline)
ET Geowizard
Tool
Multiple
Polygons Merge Dissolve
Sub-alpine Altitudinal Belt (Polygon)
Montane Altitudinal Belt (Polygon)
Alpine Altitudinal Belt (Polygon)
Clip Erase
63 d
b
g a
e c
f
Figure 3.5: Process for developing the t hree altitudinal belts of the Drakensberg Park; a) management compartment with contour lines, b) closed polylines, c) several different polygons, d) different polygons merged to create individual belts, e) alpine and montane belt polygons clipped to UDP_WHS boundary, f) clipped sub-alpine belt, g) three altitudinal belts.
64 Figure 3.6: A management compartment; a) before union; b) after the union subdividing the compartment into various belts.