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Scale and Purpose

Here are some thoughts from Andy about what happens when we change the display scale.

 

What is the difference between a chart used at 1:1,000,000 and one used at 1:10,000?

The most obvious, for charts of the same size, they cover very different amount of area.

The actual numbers are staggering. Consider a chart drawn at a 10th of the scale – a length on the ground is displayed ten times shorter. This is ten times less detailed on both axes – one hundredth of the area. An area that was represented on a screen of 1 million pixels (a typical laptop) now has to be represented in only 10,000 pixels! That is a serious reduction. It is just 100 pixels by 100 pixels.

Image at 1:10,000clip_image002

Image of the same geographic area at 1:100,000

clip_image004

Incidentally, the range of useful scales for marine charts is about 1:2,000 to 1:20,000,000. That is an area ratio a million times greater than the above example.

The less detailed one has to leave a lot out. Otherwise it would be a sea of overlapping symbols.

How do we choose what to leave out?

The cartographer does this.

Traditionally, paper charts at different scales were quite separate. The cartographer drew each one by hand, more or less independently. Before even getting to the point of creating detailed charts, lots of decisions have already been made about which areas to even bother with. The mid oceans are only charted at 1:3,500,000 or worse because it doesn't help the mariner to have more detail. There is nothing to hit. It is 7, 14 or even 28 days to land. The detailed charts were only created for navigationally interesting places, ports, narrow channels, the coast because we don’t want to hit it.

With electronic vector charts, the attraction from the chart producer’s point of view is to have a single database of chart data and publish it at different scales – thus reducing the number of layers of chart that he / she has to maintain. In principle, you might feel that this approach allows you to produce a chart at any scale. This is re-enforced by the ability in many navigation programs to display the chart at any scale. But I don’t think this really works.

For a start, the detailed information required to make detailed charts still only exists for the interesting areas. Without that information, a detailed scale chart doesn’t give you any more information, it is just bigger. This is exactly the same as with town street maps. Once the scale is detailed enough to include all the roads and their names, you don’t gain anything – that bumper sized street guide just uses more paper to display the same information in a more spaced out way.

Continuing with the road atlas theme, we have different types of map for different purposes – from national freeways, through state and county maps to individual town plans.

Navigation charts are just the same. In ENC vector charts we have these navigational purposes

Overview

General

Coastal

Approach

Harbor

Berth

Their purposes are, hopefully, fairly obvious from their names. They have a very commercial ship feel to them though – were does that beautiful cove with a sandbar across the entrance fit into the scheme? – it needs a detailed chart, but isn’t the sort of place that commercial shipping cares about. Well, in US waters, Approach is actually more or less continuous along the coastal strip. Harbor also covers a lot more than just harbors, covering a substantial fraction of the coastal strip. So really the navigational purposes are more like scale bands.

Paper charts are also purposed, though, as they are selected manually, it wasn’t so explicit – you just pulled the relevant ones out of the chart drawer - a passage chart for the middle and a harbor chart for each end.

Now look at an electronic chart display. How do we choose the right chart, the right navigational purpose now?

Well, it is a trick question, because typically, you don’t; the computer decides. It switches between purposes as you zoom in and out. This explains why the display changes by more than you expect for some zoom steps. It has just changed navigational purpose.

It is obvious that this will not always do what you want. The computer does not fundamentally know what your current navigational aim is – so as you sail along the coast, it will tend to display harbor charts for the ports you are merely passing by.

The alternative would be to choose the navigational purpose manually – so you always get what you ask for (which might not be quite the same as want you want, or even what you need).

Now we can explain why we don’t do that.

Assume you have a button for each navigational purpose. Say you press “Approach”. The computer displays Approach ENC only. If you are zoomed in to a scale of 1:2,000, you will see a small part of a very coarse chart. If you are zoomed out to a scale of 1:1,000,000 you will just see a few small areas of mush – chart that is so scrunched up you can’t read the detail it contains – and most of the screen will be blank because approach data only exists for the coastal strip. At some scale in-between, you will see a useful Approach chart.

Assuming that we don’t think that computers suffer when we make them work harder, the computer can do much better than this – it can automatically display the less good navigational purposes underneath the approach chart. And it could indicate where there are areas of a more detailed navigational purpose available.

In practice, indicating the areas where there is more detail does not help much – it is usually obvious from the chart where the coast is – that is where the more detailed purposes will cover. There is little point drawing a more detailed navigational purpose when the scale is such that it will be too scrunched up to be legible. It is better to display a less detailed navigational purpose that is nearer to the display scale. So the computer ignores ENC data that has a much more detailed compilation scale than the current display.

In contrast, it is always worth displaying the less detailed purposes underneath because they will still give some context however un-detailed they are – after all, blank areas of screen give no information at all.

Given these rules, if you press the button for the most detailed navigational purpose, Berthing, the computer can automatically display the best ENC data that is available for the current display scale.

In Nuno we decided that this was want you would want 90% of the time, so the complication of having all those buttons was not worth it. Pressto – an automatic system that almost always does want you want, and on the occasions when ideally, you would prefer something slightly different, it errs on the side of safety, by displaying a slightly more detailed chart.

Navigational purposes are the main reason why the content of the chart changes at certain scales. As you zoom in to a scale where it is worth displaying a more detailed navigational purpose, the chart is completely replaced by the new navigational purpose; it might be from a different survey; certainly the person who compiled the chart will have included more detail, both in objects such as buoys and in the wigglyness of the coast and depth contours.

For what it is worth, here are the scales of each navigational purpose…

Nav Purpose

Normal scale range

Outliers

Overview

1:1,100,000 to 1:4,860,000

1:10,000,000

General

1:500,000 to 1:1,200,000

1:1,444,000

Coastal

1:180,000 to 1:600,000

 

Approach

1:80,000 to 1:120,000

1:52,150

Harbor

1:10,000 to 1:50,000

1:72,000

A few at 1:5,000

Berth (only 2 cells)

1:2,500

 

Because there is such a wide range of scale within each navigational purpose, it is possible to find an area and scale combination where some of the chart is displayed at one navigational purpose and the rest at the next one. This results in a discontinuity in the chart at the boundary because of the different surveys and detail level in the two purposes.

To summarize this discussion. The software tries to display the best detail available for the current scale. Knowing about the mechanisms involved in the choosing the data to display allows you to understand the discontinuities that sometimes appear and make the best use of the electronic chart.

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