# calcArea: Calculate the Areas of Polygons In PBSmapping: Mapping Fisheries Data and Spatial Analysis Tools

## Description

Calculate the areas of polygons found in a PolySet.

## Usage

 `1` ```calcArea (polys, rollup = 3) ```

## Arguments

 `polys` PolySet to use. `rollup` level of detail in the results; `1` = `PID`s only, by summing all the polygons with the same `PID`, `2` = outer contours only, by subtracting holes from their parent, and `3` = no roll-up.

## Details

If `rollup` equals `1`, the results contain an area for each unique `PID` only. When it equals `2`, they contain entries for outer contours only. Finally, setting it to `3` prevents roll-up, and they contain areas for each unique (`PID`, `SID`).

Outer polygons have positive areas and inner polygons negative areas. When polygons are rolled up, the routine sums the positive and negative areas and consequently accounts for holes.

If the PolySet's `projection` attribute equals `"LL"`, the function projects the PolySet in UTM first. If the PolySet's `zone` attribute exists, it uses it for the conversion. Otherwise, it computes the mean longitude and uses that value to determine the zone. The longitude range of zone i is -186 + 6i < x <= -180 + 6i degrees.

## Value

PolyData with columns `PID`, `SID` (may be missing), and `area`. If the projection equals `"LL"` or `"UTM"`, the units of area are square kilometres.

## Author(s)

Nicholas M. Boers, Associate Professor – Computer Science
MacEwan University, Edmonton AB
Last modified `Rd: 2013-04-10`

`calcCentroid`, `calcLength`, `calcMidRange`, `calcSummary`, `locatePolys`.

## Examples

 ``` 1 2 3 4 5 6 7 8 9 10``` ```local(envir=.PBSmapEnv,expr={ #--- load the data (if using R) if (!is.null(version\$language) && (version\$language == "R")) data(nepacLL,envir=.PBSmapEnv) #--- convert LL to UTM so calculation makes sense attr(nepacLL, "zone") <- 9 nepacUTM <- convUL(nepacLL) #--- calculate and print the areas print(calcArea(nepacUTM)) }) ```

### Example output

```sh: 1: cannot create /dev/null: Permission denied

-----------------------------------------------------------

PBS Mapping comes with ABSOLUTELY NO WARRANTY;
for details see the file COPYING.
This is free software, and you are welcome to redistribute
it under certain conditions, as outlined in the above file.

A complete user guide 'PBSmapping-UG.pdf' is located at
/usr/local/lib/R/site-library/PBSmapping/doc/PBSmapping-UG.pdf

Packaged on 2018-06-05
Pacific Biological Station, Nanaimo

All available PBS packages can be found at
https://github.com/pbs-software

To see demos, type '.PBSfigs()'.
-----------------------------------------------------------

convUL: Converting coordinates within the northern hemisphere.
PID         area
1       0 5.573726e+05
2       1 6.300381e+06
3      15 8.967819e+04
4      33 3.207198e+04
5      39 6.813302e+03
6      47 9.310090e+03
7      60 1.006880e+04
8      67 1.041737e+04
9      68 4.230791e+03
10     74 5.406475e+03
11     88 4.375389e+03
12     89 1.997024e+03
13    109 3.235804e+03
14    133 1.967937e+03
15    148 2.667601e+03
16    155 2.833465e+03
17    158 2.977172e+03
18    173 4.666767e+03
19    178 1.322215e+03
20    192 8.303504e+03
21    225 1.485232e+03
22    228 6.858324e+02
23    263 1.037473e+03
24    272 5.388982e+03
25    281 9.365769e+02
26    292 9.032412e+02
27    296 3.021929e+02
28    347 2.121584e+03
29    375 5.433573e+02
30    384 3.395853e+02
31    389 4.661454e+03
32    390 5.651404e+02
33    394 4.903152e+02
34    397 8.575281e+02
35    440 2.211727e+02
36    453 1.259021e+03
37    480 2.410199e+03
38    501 3.903431e+02
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51    650 3.872434e+02
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55    726 3.994991e+02
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57    850 5.524838e+02
58    880 1.795207e+02
59    889 1.496277e+02
60    893 1.226877e+02
61    896 4.149567e+02
62    914 1.415734e+02
63    929 1.824923e+02
64    935 5.569482e+02
65    939 1.908028e+02
66    952 1.863538e+02
67    969 1.122716e+02
68    983 1.749225e+02
69    995 3.363498e+02
70   1035 1.485838e+02
71   1048 2.537314e+02
72   1070 8.275666e+01
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76   1101 1.925266e+02
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```

PBSmapping documentation built on Jan. 15, 2021, 3:41 p.m.