Original subtitles

Scale on a web map is a little different than it would be on a paper map.

So, let's have a look at scale in relation to web mapping.

Okay. So, here we have a zoomed out map of the entire world in ArcGIS Online.

This is actually referred to as zoom level zero.

What this is, is that it's almost the entire world.

It's a little bit of the poles that are cut off,

but almost the entire world is found inside this one square,

that's 250 pixels wide by 250 pixels high.

So, it's a perfect square which happens to be very

useful for making web maps as you'll see in a second.

If I zoom in, what's just happened here is that the map scale has gotten larger.

In fact it's doubled.

So, that the size of the map that we're looking at has doubled,

and so we've increased the map scale.

If I zoom in, again,

we've gone to another zoom level or scale level.

That's the terminology that's used for web mapping,

and the map scale has gotten larger again and it's doubled again.

So, this is zoom level three.

We can continue to zoom in here and see more and more detail.

If we zoom in all the way to the highest or largest scale map,

which is zoom level 20,

you can see individual buildings,

the footprints of buildings,

and you may recognize this or may not depending on if you've been there before,

it's Buckingham Palace in London England.

So, if you ever have a chance to go there,

it's an absolutely beautiful impressive building.

I took a tour through there one time.

I think it's open to the public, just in the summer when the Queen is not in residence,

and I would recommend it,

it's really interesting to go through.

Anyway, so that is the highest level of zooming in,

if you want or the largest map scale that's available in a web map.

Just to give you a sense of,

so, we went from the entire world,

down to one building which is incredibly impressive to say the least.

So, up until now,

I've been talking about maps as though they're static like you would have on a paper map.

But it's a little bit different when you're talking about a web map,

because that might be seen on a big screen or a desktop computer,

it might be on a tablet,

it might be on a phone.

What's interesting about this is that,

the area that's being mapped here on this screen is the same.

In other words, they're all showing the same amount of the world.

Right? But the size of the map that's being used to show it is different.

So, this is actually a larger scale map here,

and this is a smaller scale map here.

Because remember, the scale is just the ratio of the map distance to the ground distance,

and so the ground distance here in a way is the same,

that's what we're holding constant is that,

the distance from this side of the map to the other on the ground is the same,

but the distance on the map itself is different

depending on the screen that's being used to display it.

So, you may notice when you're looking at a web map,

that they don't include a representative fraction on those maps,

because it would constantly be changing and they'd

have to keep recalculating it all the time,

and it would have to know the size of the display that's being used and so on,

so, instead what they just do to keep things simple is,

they put a bar scale on the map, and so,

if you are looking at that on a larger screen,

the same-sized area that is,

the bar scale will automatically be larger along with the rest of the map,

and so that will still be representative of the scale of that map.

If you're on a smaller screen,

the scale bar will get smaller,

and so that's kind of a trick they use in order to be able to

actually show you what the scale is in terms of distances on the ground,

but they're not showing you that fractional version of the scale.

So, this is just a different way of thinking about scale is on the top part here,

we're keeping the distance on the map the same,

and changing the size of the map itself,

and on the bottom here,

we're keeping the size of the map the same in the distance on the ground is changing.

So, either way, it's the same thing.

Just remember it's just a ratio of map distance to ground distance,

and so you're just trying to figure out,

what is it I'm looking at and what's the relationship.

I think it's useful to know a little bit about web mapping,

this does relate to scale,

but also just about like, how does this work?

Why is it set up that way?

Because I think most of,

all of us use web maps a lot,

so, you might as well have a sense of like, well, how does this work?

So let's have a quick look.

So, here I'm showing you the entire world in one square which is at zoom level zero.

So, that's as far out as we can zoom in a web map.

When that's represented on our browser or whatever app that we're using,

if we're at the correct resolution,

then that's a 256 pixel by 256 pixel size image.

So, literally 256 pixels,

that way and that way.

Pixel by the way is a short form for picture elements.

I don't know if you knew that little trivia for you.

So, that's where the word pixel comes from.

So, there's 256 dots on each of

the axes that are being used to make up this image of the map.

So, this 256 by 256 image is referred to as a map tiles.

So, we can and show the entire world in one map tile at zoom level zero.

If we go to zoom level one,

we've doubled the size of our image of that world,

and so now we have 512 pixels to work with on both axes,

and we have four map tiles.

So, each of these map tiles is still 256 pixels this way and that way,

but since we have four tiles that's 512 pixels

all the way from top to bottom and 512 from left to right.

So, now we've doubled the number of pixels that are needed in both directions,

so that's actually quadrupling,

the number of pixels in total that we need

quadrupling the amount of data that needs to be stored,

as we increase the mass scale.

When a company like Esri or Google or whomever decides to make a web map,

what they're doing is, you're saying, Okay we're going to take the entire world,

and we're going to keep these different zoom levels available to people,

and for each one of those zoom levels as people zoom in the amount

of data that's needed to store that is going to quadruple each time.

Right? All of those map tiles at all of those zoom levels are being

stored on the servers of the company that's providing this web mapping service.

So, in this case,

it could be Esri's cloud servers that are storing that data.

So, when you are on your device,

whatever it is: phone, tablet, laptop,

desktop doesn't matter, and you're using their web servers,

what's happening is that,

when you ask to see a map of a certain area it's taking that request and sending it

to the web map providers

server and saying this is the part of the world that they're interested in.

So, then it determines which tiles are needed at what map scale,

and only sends you those tiles,

and so that dramatically cuts down on the amount of

data that needs to be sent at any given time, so,

of course if you've got a limited amount

of data on your data plan or bandwidth or whatever,

might really speeds things up and that way the web mapping app is

much more responsive and faster in terms of you being able to use it.

So, that's why it's set up that way.

Incidentally, if you're trying to get a sense of

the amount of data we're talking about here,

if you had to store all of the map data for

the entire world at all zoom levels on your phone or let's say it's your phone,

you would have to store 15 thousand terabytes of data.

So, that's an enormous amount of data,

phones are pretty fast, but they're not

pretty powerful and lots of storage but not that much.

So, that's why they have to come up with these strategies in order to

only send you the data that's needed at any given time.

But imagine having to store all that data even on a web server or Google or whatever,

I mean all of these big companies.

But that's a lot of data.

Anyway, that's how web mapping works in terms of dividing up the world into

these tiles so that they can

scale and then you can zoom in and just see the areas that you're interested in.

So, if we're looking at a web map,

and we're trying to figure out what's the scale of that map,

and if that varies depending on the device that we're using,

or even just the size of the web browser on our desktop or whatever,

if that's kind of dynamic or changing all the time,

how do we calculate what the scale is of a map?

Well, I'll just give you a quick demonstration of how this works.

If you have a map at a given display size,

you can measure it just like you would if it was a paper map.

It doesn't matter whether it's on your tablet or your phone.

Whatever it is that's the size of the area that's being mapped,

we can just use a quick calculation to figure out the ratio

of the map distance to the ground distance,

and then we can calculate what the map scale is for that.

So, we're just kind of doing

the reverse calculation that we would normally do to figure out the distance,

now we're going to use it to figure out the calculation for the scale.

Right. So, if we are looking at a distance that we're interested in,

and we want to know what the scale is for a particular area,

if we know that there's a scale is a ratio of map distance to ground distance,

and if I measured on my map that the distance for this blue arrow is 9.3 centimeters.

Okay? Now, that just means,

as long as you don't zoom in or out,

whatever that, you just kind of left your map alone,

and you've measured that distance,

at that given moment at that display,

that is the map scale,

so you've got 9.3 centimeters.

So, if this is an ArcGIS Online,

I just use the measure tool to click on one end to the other.

So, that's what is being shown here.

So, the distance from here to here,

and I just measured it with a measuring tool.

That tells me that the distance is 69 meters.

Okay. So, if we know that the distance on the map is 9.3 centimeters,

if we know the distance on the ground is 69.0 meters, what's our map scale?

Well, we just do one over x equals 0.093 meters over 69.0 meters,

remember you have to use the same units in the numerator and the denominator,

and then we solve for x and that gives us x equals 741.9.

So, now we know that the map scale is one to 742,

which is actually very large map scale.

So, that's it. That's how you can calculate map scale.

I don't know how often you would need to do it.

Often, if you're using a web map,

there's other ways to determine distances and things.

But I think it's a useful if nothing else,

is kind of a good mental exercise to make sure that

the idea of map scale and how to calculate it is clear in your mind.