Index contours help the map reader obtain a far better feel for the topography of one area. Symbolising index contours is simple to perform with a small bit of preparation. 

In an earlier write-up we looked at producing elevation contours:

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This time we’ll watch at just how we can much better display our border by symbolising some index contours. If you’ve spent any type of time through topographic maps you’ll be acquainted with this – at some multiple the the contour interval (the upright distance between contours) we usage thicker present to to mark the elevation change. On our 1:50,000 range topo maps through a contour interval the 20 m, this index contours are supplied every 100 m, as shown below:

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Every 100 ms above sea level, the edge is a darker, thicker brown and is additionally labelled through the elevation, when the others space lighter, thinner and have no label. So the trick right here is come easily be able to identify those contours and also then offer yourself an attribute to differentiate between the two.

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For this task we’ll add a brand-new field to our border attribute table, add a default value of 0 to every the records, then pick the index contours and also give castle a value of 1. Naught special around 0 and 1; they’re just convenient. We can just as quickly use A and also B or Bret and Jemaine. The said, let’s use Bret and also Jemaine instead.

Just as a reminder, here are the border I obtained earlier:

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And here’s a fast look at my attribute table:

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I’ve gained the contour attribute highlighted and you can see that they room multiples of 20 m. An initial step is to include a brand-new field. Provided the surname I’m using, it will be a text field and also only requirements to be 7 characters long (“Jemaine”) but I’ll do it 10 to it is in safe. Native the table menu I’ll choose “Add Field…”

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I’ll speak to that ar “Index” set the type to Text and make the size 10.

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Next, I’ll execute a field calculation to add “Jemaine” to all the areas as the default value. Later I’ll come earlier and adjust the entry because that the table of contents contours:

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(NB: once using the field Calculator to add text, don’t forget to placed your entry in quotes.)

Next, we can choose those contours that space multiples the 100. This is a little enough layer the if us really wanted to we can just carry out it manually through sorting our edge values and also only changing the ones we require to. We’ll use pick by Attribute to present how we can do this for larger datasets. Currently how have the right to we effectively use a questions to pick out every the contours who elevation value is a many of 100?

Those of girlfriend who have done any type of coding will likely have actually come throughout the modulo function. Put very simply, the modulo role gives friend what continues to be after division. Therefore let speak we divide 5 by 2, which is of course 2.5, or 2 and 1/2. The remainder is 1 (since 2 x 2 = 4 v 1 left over). In contrast, 9 separated by 3 is 3 through no remainder. Another method to write this is 5 mod 2 = 1 and 9 mod 3 is 0. We have the right to use this logic to select all our border that have actually no remainder when split by 100. (Hope that makes sense…). We can use this is the choose by Attribute home window in this way:

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The an essential phrase below is: MOD(Contour, 100) = 0. The is a true/false statement and also all the true results will gain selected. Read this together (roughly): “Divide every Contour values by 100 and also select those that have 0 as a remainder”. Here’s the result:

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Now we’ll use the ar Calculator to adjust the worth of table of contents for every the selected documents to “Bret”

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Right – that’s the difficult part. Currently we’ve got sufficient information to start symbolising things.

In the layer’s properties > Symbology tab us should set Show to categories > unique values, collection the Value field to Index and click “Add every Values” (don’t forget come untick ):

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To set these up we’ll aim to recreate the colours used on the 1:50K topo maps. We know that the Bret values have to be our index contours while the others are just your consistent run of the mill contours. I’ll do those persons thicker and also darker brown and the others the the opposite (poor Jemaine, that won’t prefer that).

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With this result:

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Well that’s not also bad, eh? I might think around softening the contours a little by one of two people making this layer a bit much more transparent or slighting reduce the thickness – they’re illustration a little bit too lot attention come themselves ideal now, yet I’ll worry around that later.

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I think this has actually gone on quite long sufficient so I’ll covering the labeling side of things in an additional post. Those of friend still awake have the right to tune in next time for the dramatic conclusion to our contouring efforts.