Coastline length – Part 2 (data analysis)

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Now that I had a municipality attribute in my coastline dataset, I was able to calculate the length of each segment and used Excel to summarize the calculations per municipality.

Scale-dependant result

To double check that my results were coherent, I looked for official figures online. I knew already the information didn’t exist for all municipalities, since I had done that search before doing my calculations, but some municipalities separately publish their figures. Vordingborg, for instance, is the municipality with the longest coastline, which was also what I had found. However, on the official website of Frederikshavn (one of the first municipalities to be updated in Trap), the figure given is 105 km.. while I had calculated 163 km. Actually, there are so many islands, that the length of the continental coastline is only 150 km. The dataset I used was the highest accuracy available; so I decided to do the same calculations with the other scales: 1:500,000 and 1:20,000,000, to see if the ‘official figure’ was taken from one of these datasets.

Exploring these differences

As expected, the other scales did not have as much coastal details, and some islands were not included. The 1:10,000 scale included every single turns and meanders in the coast, including the crenels on sea defences! As a results, for most municipalities, the higher the scale, the shorter the coastline. There were also some parts that can be considered as an aberration when looking at the coast: for instance in Odense the entire estuary was included, making the coast much longer at the 1:10,000 scale than the others.
Odense
Coastline of Odense – in green at the 1:10,000 scale and in red at the 1:500,000 scale
The harbours also are not always detailed in the same way, as shown here in Frederikshavn:
frederikshavn3frederikshavn2
When included in the higher scales, the details of the coast of the islands can be quite different. For instance, Christiansø (one of the islands furthest east of Denmark, in the Baltic Sea) has a very different coastline depending on the scale.
christiansø

Finally, there were also issues of points density, like in Frederikshavn (again!):

frederikshavn

In both scales we can see aberrations: at 1:10,000 (in black), we see the crenels of the sea defences in the southern most coast; while at the 1:500,000 scale, there are obvious parts of the coast that have been generalised.

Conclusion

The differences between the length at different scales is therefore:
  1. the inclusion of some features of the coast (estuary, coast, islands, ..)
  2. the density of points used to draw the coastline

It is very difficult to find any authoritative figure. The danger in choosing a scale with very fine details is that we eventually measure something that does not exist. In Frederikshavn, even some of the sea defences crenels are included. Is that really the coast? But selecting a scale that does not show what exist is neither satisfactory, since it hides some real coast.

Appendix

Here is the table of results of my calculations:

KommuneLength 1:10.000 (km)Length 1:500.000 (km)Length 1:2.000.000 (km)
Vordingborg398339321
Guldborgsund332281263
Lolland307267254
Thisted258233222
Sønderborg258239232
Ringkøbing-Skjern252205192
Aalborg232192182
Slagelse222182176
Skive207187175
Morsø201170156
Kalundborg189169160
Svendborg189163151
Bornholm188122116
Langeland188168159
Læsø168114101
Frederikshavn163120117
Syddjurs161134129
Odsherred160139137
Norddjurs157127124
Nordfyns155122110
Kerteminde153132128
Lemvig152122117
Tønder121121110
Frederikssund150110100
Struer149137128
Jammerbugt139121112
Haderslev138121108
Samsø137114105
Faaborg-Midtfyn130110105
Mariagerfjord12310397
Ærø123106100
Middelfart120107105
Esbjerg1198785
København1187270
Randers1159788
Tårnby1126046
Aabenraa736663
Vesthimmerlands11010394
Assens1049188
Holbæk1038178
Næstved1037775
Odder987773
Holstebro917666
Varde877675
Hedensted807469
Horsens766563
Aarhus735655
Fanø735149
Lejre735754
Halsnæs644544
Hjørring635352
Kolding635957
Nyborg564746
Roskilde503835
Fredericia484039
Stevns444040
Viborg443938
Helsingør352625
Odense351211
Vejle343029
Faxe343029
Gribskov323131
Køge252121
Dragør201414
Brønderslev181312
Greve151212
Fredensborg1399
Hvidovre121010
Solrød1288
Gentofte1088
Rudersdal966
Hørsholm977
Christiansø944
Lyngby-Taarbæk433
Ishøj487
Brøndby389
Vallensbæk033