How to do Area calculations in Tableau [ with correction ] : New in Tableau 2021.2

0:00Hey, it's Tim here today, different kind of

0:02video, I'm actually uploading a correction.

0:04Yes. When I

0:05made a video about the error calculation in

0:072021.2, there was essentially a mistake

0:10that I made, and it

0:11was an assumption. Now what actually

0:13happened in this error calculation is I had

0:15a data set

0:15from a London data provider. And in that

0:18data set that actually pre calculated the

0:21area. And the

0:21reason I was looking for this data set is

0:23because I needed some way of validating

0:25tableau's error

0:26calculation against another data set. So

0:28the assumption I made was that the company

0:30that

0:30produced this data had done it correctly.

0:33Now it actually turned out that when I did

0:35this comparison

0:36in the video, you can see here at seven

0:38minutes 55, I'm actually comparing seven

0:40side by side,

0:41the two calculations, and you can see that

0:43I'm calling out that the tablet error

0:45calculation

0:46is way off, actually sent this feature into

0:48tableau. And they actually took some time

0:50out

0:50to look at the data set and to look at the

0:52error calculation. And it turned out using

0:54another tool

0:55that the error calculations in the data

0:57source I was using was actually incorrect.

1:00And therefore,

1:00I came to the conclusion that the error

1:02calculation was not as accurate as it

1:04should be

1:05over certain types of geographies. And in

1:07some cases, you can see here side by side,

1:09it maps pretty well. But there was just a

1:11few object IDs here that were generating

1:14areas that just weren't matching. And that

1:16wasn't the fault of tableau is actually the

1:18fault of the

1:18error calcs that were done in this data set

1:21. Now, it is really important lesson, which

1:23is to never

1:24make the assumption that the data source

1:26that you find from a trusted source is

1:27always correct.

1:28And number two, I jumped to a conclusion

1:30without first validating it before I

1:31released the video.

1:32Now in order to fix this, what I'm actually

1:34going to do is I'm going to play the video

1:35again,

1:35I'm not going to re record the video. But I

1:37will put a note up in the section where I'm

1:39calling out

1:40the mistake or the issue with tableau's

1:41data source to actually validate validate

1:43that the

1:44issue here is not to do a tableau. And I

1:46sort of don't want to re record the video

1:48and then delete

1:49the old one because I think it's good to

1:51sort of fess up to your mistakes. And

1:53actually realize that

1:53we're all still learning. And at the end of

1:55the day, tableau did the due diligence and

1:57to make

1:58sure that the calculation was doing the

2:00best job it can. Now, if you're into some

2:03sort of detail

2:04about this, it's actually quite hard to do

2:06error calculations on something like the

2:08curvature of

2:09the earth, depending on the size of

2:11geography, you can get slight variances

2:13depending on how you do

2:14the calculation. So I'm not the expert on

2:16this, I'm going to maybe link to some

2:17articles in this

2:18description for this video. So you can go

2:20off and find out the complexities of doing

2:22that there's

2:23actually a tableau paper on hexagons and

2:25hex mapping and sort of showing the

2:27disproportionate

2:29behaviors that you can get and how you have

2:31to sort of correct for that over large

2:32areas. So I

2:33actually recommend you read that that's a

2:35proper in depth paper by people who know

2:36what they're

2:37talking about. And that can give you more

2:39context into this. But that's pretty much

2:41it for me,

2:41I just wanted to add this correction in

2:43front of the video, but upload the video as

2:45is rather

2:45than trying to sort of hide the fact that I

2:47got it wrong. So there we go, we all make

2:49mistakes. And

2:50hopefully you can learn from mine. Thanks

2:52for watching, and I'll catch you in the

2:53next video.

2:54Hey, Tim here in today's video, I'm taking

2:56you through a new feature in 2021.2. And

2:59that is the

2:59new area function, a new spatial function

3:02that allows you to calculate the area of a

3:04polygon.

3:04But before I get into this, be very careful

3:06with this. If you use this over large ge

3:09ographies,

3:10it's actually only giving you an estimate.

3:12I'll get into why that is in a second. Let

3:14's get stuck

3:15into this. So the first thing I'm going to

3:16do is I've actually gone to this website,

3:18when you're

3:18working with spatial data, it's always good

3:20to have a good sample data. And I've gone

3:22to this

3:22London data store to grab flood risk zones

3:25as calculated by the Environment Agency.

3:28Essentially,

3:28they figure out where there's a flood most

3:30likely to happen in London. And they've

3:32created a shape

3:33file or a bunch of polygons for us to use

3:35if we want to visualize that it's part of

3:37an open data

3:38initiative. Now, this was created three

3:40years ago, so it's nothing new. But I'm

3:41going to connect to

3:42this directly in Tableau, I've actually

3:44gone ahead and downloaded it to my desktop,

3:46so it's already

3:46there. So I'll just go back into Tableau

3:48here, and we'll just go ahead and use the

3:50spatial file. So

3:51if you've never used a spatial file before,

3:53it's actually this option here, it's

3:54essentially a type

3:56of file that has information about

3:57geography, that's the simplest way to sort

4:00of think of it.

4:00And so if I click on this, you'll see that

4:02I actually have the ability to open the

4:04file and

4:05shape files don't just come on their own,

4:07they tend to come with other supporting

4:08files. But the

4:09one file that we're interested in is this

4:11one, which ends in dot s HP. So if we click

4:14that and

4:14click open, you should see that Tableau

4:16handles this absolutely fine. It's been a

4:18native capability

4:19of Tableau to support these out of the box.

4:21So it's just right into this Tableau. And

4:23the great

4:24thing is that it works with everything

4:26including the data models. If you want to

4:27pair this up with

4:28other days that you have and find out where

4:30the properties are in certain London flood

4:32zones,

4:32you could it's an absolutely very simple

4:34thing to do. Maybe that's for another video

4:36. But

4:37nevertheless, we've connected to this data.

4:39Now I'm just going to go right into sheet

4:41one. And

4:41you can see that here on the left hand side

4:43, we've got all our fields. Now in Tableau,

4:46it's super

4:46easy to create a map, you just double click

4:48any geographical field, and it will do it

4:50for you.

4:51Where I know it's geographical is because

4:53there's this world icon to the left. So let

4:55's just double

4:56click that and see what happens. It's a

4:58geometry field and boom, we already have a

5:00map. Now, I know

5:01this is London, but it has some really poor

5:03context on here. There's nothing on the map

5:05that suggests

5:06this is London. So let's customise this map

5:08before we carry on. Let's go to map layers

5:10at the top.

5:11Let's choose a more interesting map, let's

5:13choose streets. And this should start to

5:16give us a little

5:16bit more sort of a vibrant map. I'll sort

5:18of dial down the the wash in the background

5:21so that the

5:22visualisation is actually what we see

5:24prominently. But it's nice because I have a

5:26literal label that

5:27says London, have all the London boroughs

5:29and towns. So if you're familiar with

5:31London, this

5:31should start to look very familiar for you.

5:33But nevertheless, we're here to look at

5:35this new

5:35function. So I'm going to close this map

5:38layers option. And when I look at this data

5:40, if I just

5:40open this little data preview for that, I

5:43just went over here, that clicking that

5:45little table

5:46icon opens up this data preview that you

5:48can see. And you can see that there's a

5:49bunch of different

5:51shapes, and each one has an object ID. So

5:53what I'm going to do is I'm actually going

5:55to use that

5:55object ID. Now, the other thing to bear in

5:58mind is that I've actually already got the

6:00area in here.

6:01And the reason I want this in here is

6:02because I want to show you how accurate

6:04this area calculation

6:05actually is. In essence, the bigger the

6:08proportion of area, the less accurate this

6:10is going to be.

6:11And we'll kind of do a quick calculation

6:12just to see how much it sort of varies. So

6:15let's,

6:15let's go ahead and look at that. So let's

6:17go ahead and let's just close this. And now

6:20what I'm going

6:21to do is grab objects ID and just put it on

6:23colour so that each and every one of these

6:25polygons gets

6:26its own sort of colour. Now, the reason

6:28this isn't quite showing what I wanted to

6:30show is because I

6:31need to bring in more context. So what I'm

6:33going to do is I'm going to actually drag

6:35the object ID

6:36up into this table section. What's

6:38happening here is that the object ID is

6:40being treated as a number.

6:42So it's actually aggregating it. I don't

6:44want that. I want it to treat it as like a

6:45text field. So

6:47what I'll do is I'll cheat on it a little

6:48bit. I'll actually drag object ID up here.

6:51You'll see

6:51that it changes two dimensions when I do

6:54that. And now it's blue. And now what I can

6:56do is put

6:57that on colour. Now each and every one of

6:59those shapes has its own unique colour. And

7:02if I hover

7:02over that, you can see that it sort of

7:04activates and we're now able to click on

7:05them and interact

7:06with them. So that's working really, really

7:08well. Now, the other thing I'm going to do

7:10is I'm also

7:11going to just look at this table again, and

7:13just see what's also useful to bring

7:14probability band.

7:15That's probably telling you how much of a

7:17flood risk zone this is. So what I'll do is

7:19I'll put it

7:20on a tool set because I want this in the

7:22tool tip when I hover over something. So

7:24you can see that

7:25some of these are very low, some of these

7:27are very high. And it essentially changes

7:29depending on the

7:30part of the map that we click on. But it's

7:33sort of working. Anyway, we've got enough

7:36information

7:36on our visualisations. Let's go ahead and

7:38actually use this function. So it's a super

7:41simple function.

7:41It's actually one of these functions in

7:43Tableau all together. So if I just type in

7:46area, you see

7:47that you get a little hint here of what you

7:49should use. So you should use a polygon

7:50dimension in here

7:52and the units that you want to measure the

7:53area in. So in this case, my polygon

7:55dimension is this

7:56geometry field. So I can actually just drag

7:59it in and drop it right there in Tableau,

8:01then hit a

8:01comma. And I'd like this done in metres. So

8:04I'm just going to do two speech marks

8:05single or double

8:06works perfectly fine. Enter an M and then

8:09now that's our calculation done for metres.

8:11You can

8:12also do kilometres and miles, you just have

8:14to find the appropriate documentation for

8:16that. In

8:17fact, let me bring that on screen now

8:18actually, you know, when I do these videos,

8:20I literally

8:21have the documentation to the left of me.

8:23So you can see here, the different

8:25dimensions you can use

8:27so metres, kilometres, miles and feet,

8:29those are all the different syntaxes you

8:31can use. So you

8:32could actually probably use a parameter to

8:34switch between those. So you can calculate

8:36all of them

8:38and then just put a parameter in here to

8:39change metres to feet and then it will

8:40dynamically

8:41recalculate it rather than using a

8:43parameter to change the different

8:44calculations. But nevertheless,

8:46this is really cool. And we've got the area

8:49in metres squared, I just put empty as a

8:52shorthand,

8:53and we'll hit apply. And now what I want to

8:55do is I want to grab the area in metres

8:56squared and the

8:57shape area and put them both on the tooltip

8:59so we can actually see what those two are

9:01like. Okay,

9:02now that's working, all the calculation is

9:04done. And now when I hover over this, you

9:07can actually

9:07see the two values, you can see here area

9:09in M2 and the shape area. Now there are

9:12subtle

9:12differences, okay, and we're talking

9:14roughly 1000 metres squared here or there,

9:17depending on how big

9:18the geography is. If I go down here to a

9:21slightly smaller one, you'll notice that

9:23the differences

9:24become larger, the bigger the geography,

9:26but if we go to a really small one, let's

9:28go to one of

9:28these ones here, they're very, very small

9:30differences. So essentially, the problem

9:33with

9:33this function is that the shape of the

9:36world is not flat. If you're a flatter,

9:39this is not the

9:40video for you. But essentially, the world

9:42is a curved platform. And the more curved

9:44something is,

9:45the harder it is to actually calculate the

9:47area because you've got to sort of flatten

9:49it out. And

9:49by the time you do that, it doesn't

9:51actually make a perfectly flat shape. So

9:53what this area

9:54calculation doing is sort of trying to

9:56compensate for that, but it's not really

9:58doing it sort of

9:58fully, you need a little bit more of a

10:00complicated sort of calculation in order to

10:02do that. Whereas

10:03the error calculation in this spatial file

10:05has actually been calculated by the

10:07environment

10:08agency using fairly accurate mechanisms,

10:10because they have to know the size of these

10:13things,

10:13things like insurance policies are based on

10:15these ratings. So they have to be pretty

10:17accurate as to

10:18how they work. So you can see there's a big

10:20difference. And what I'm going to do is I'm

10:22actually just going to build a very, very

10:24simple visualization, I'm just going to

10:26bring in object ID

10:27at all of them. And what I'm going to do, I

10:29'm going to bring in the area in meter

10:32squared,

10:33and I'm going to bring in the shape area in

10:34the table next to it. So you can just sort

10:37of see

10:37some of these side by side. Now, some of

10:39them are very similar, some of them are

10:42very different.

10:43For example, this one is sort of

10:44interesting, because, you know, this one's

10:47just so way off. So

10:48why is Tableau getting that so different,

10:50whereas some of these, they're almost so

10:52bang on. So it

10:53could be to do with the shape and the form

10:55of the particular geography could be a long

10:58versus thin

10:59versus, you know, you don't know all these

11:01sort of variables. So in generally speaking

11:03, you can see

11:04that this is an estimate. But it's just

11:06worth being careful and knowing that this

11:08is an estimate,

11:08because you see that this particular object

11:10, it says it's 12. But the actual area is

11:13calculated

11:13by the environment agencies 224. So that's

11:16just something to be aware of. Now, what

11:18you could do

11:19is you could actually sort of create a

11:21table. So if I was to sort this from the

11:24largest shape area

11:26to the smallest, you can see that some of

11:28these are just way off. So it's definitely

11:31one to be

11:31sort of wary of and to be really careful

11:34how you're sort of relying on this function

11:36,

11:37because it can actually throw some of your

11:39things off if you rely on it too much. What

11:41are the best

11:41use cases? Listen, if you're measuring

11:43things like playgrounds, parks, this is

11:45perfectly fine,

11:46you know, nice, simple geographies, polyg

11:49ons, shapes, simple geographies, if you're

11:51measuring

11:51the area of let's say, a river that travels

11:53over a long distance, well, that's going to

11:56be a little

11:56bit more tricky, because of course, over a

11:59longer distance, there's more curvature.

12:01And the shape

12:02area or area calculation is only going to

12:04be able to calculate it to sort of one

12:05extreme. So you

12:06will get less accuracy for polygons that

12:09span really large distances and large ge

12:12ographies.

12:13But I'm not a scientist here, I'm just sort

12:15of going off what what I see and what I

12:17know I've

12:17asked the devs about. But it's definitely

12:19one to just watch out for. And some of

12:21these I think I

12:22think there's probably just an issue with

12:23the polygons, you can see that that says 20

12:26. And then

12:26it's just completely way off. So it's not,

12:28it's not sort of an exact science. And I

12:31need to get

12:31more into this to really understand, you

12:33know, why are some of these really, really

12:35close and why

12:36some of them really, really far off. The

12:38figure on the right is the accurate one

12:40from the Environment

12:40Agency, I just wanted to get that there

12:42before I show you a calculation. And I tell

12:44you that it's

12:45correct as fact and then he has a slight

12:47margin. And actually, some of these have

12:49huge margins. So

12:50I'd be interested to see what you think of

12:51that. If you're a geographer, or you

12:53actually use spatial

12:54functions all the time, I'd love to know.

12:57And if you know anything about why this is,

12:59why is this

13:00calculation in Tableau getting it so

13:02different to the values that have been

13:03calculated by the

13:04Environment Agency, what I will do is I'll

13:06put a link to this data set and maybe you

13:08can tell me

13:09if you've got some insights to why that is.

13:11But I'd love to get to know more. Alright.

13:13Anyway,

13:14that's that's it. We have arrow functions

13:17in Tableau. Now you can use these over

13:19simple

13:19geographies and they're going to be really,

13:20really useful. Thanks for watching. If you

13:22've enjoyed

13:23this video, be sure to check out some of

13:24the other videos in 2021.2 on this channel,

13:27as well as other

13:28versions of Tableau from way past all the

13:30way to functions in Tableau. I cover

13:31everything so get

13:32in touch. Let me know what you'd like to

13:34see and if not, hopefully I'll catch you in

13:36the next one.