What is Coverage map?

A coverage map is the document people look at most often before buying, and the one most often misread. What you see is a shaded area laid over a real map, and it looks like a clear statement of where there is signal and where there is not. What it is actually showing is the output of a model rather than a set of point-by-point measurements taken on the ground. The gap between those two things explains almost every case where a traveller concludes the map lied.

This page explains how coverage maps are produced, how to read one so it is actually useful, which kinds of terrain they get wrong most often, and — for anyone using a travel eSIM — whose map you should be looking at in the first place, since a travel plan runs over partner networks in the destination country and that changes the answer considerably.

What a coverage map is

A coverage map is a picture an operator publishes to show where it provides service. It is normally split into layers by technology — a layer for the newer network generation and one for the generation before it — and shading is used to indicate expected strength, with denser shading meaning a stronger expected signal and lighter shading meaning weaker service or service under narrower conditions.

The word doing the work in that paragraph is expected. These maps show a prediction, not a confirmation. Most operators say so themselves in a line of small print at the edge of the page, stating that the map is an estimate and does not constitute a guarantee of service. That line is worth reading before you read the colours.

Modelled, not measured

The most accurate way to know the signal at a given point is to stand there and measure it, which is not possible at national scale. Operators therefore use radio propagation models instead, feeding in the position and power of each base station, mast heights, antenna orientation, and terrain data, and letting the model calculate where the signal is likely to reach and at roughly what strength.

Models do well at the broad level — they will correctly tell you that a city is served more densely than the valley next to it. They do much worse at street level and inside buildings, because the things that block radio waves at that scale carry more detail than any model can hold: a tower finished last year, the material a wall is made of, even the same trees carrying different amounts of leaf in different seasons.

Why shading does not promise a signal indoors

Most maps model outdoor signal at roughly the height of someone holding a phone while walking down a street. The moment you step inside, the signal has to pass through at least one wall, and it loses strength according to what that wall is made of. Buildings with metal-coated glass, basements, lifts, and the bathroom in the middle of a floor plan are all places where the signal can fall away sharply, even in the middle of the darkest shading on the map.

The same applies to metro systems, tunnels, underground car parks and the lower floors of large malls. Some have in-building distribution systems installed and work well; some do not, and a national map does not distinguish between them. If being connected inside a metro station matters to your plans, treat it as uncertain and prepare a fallback.

Reading the layers and the legend

The first thing to do when you open a map is find the legend, not zoom straight to your destination, because the same colour on two operators' maps can mean two different things. Some use colour to separate network generations, some use it to separate predicted signal levels, and some combine both in one scheme.

The second thing is to switch off the newest technology layer and see what remains. In many places the widest layer is the older generation, which is still perfectly serviceable for maps, messaging and search. The picture you get with the top layer turned off is often closer to real experience than the one with every layer stacked together.

Reading a map against your actual route

A map earns its keep when you read it along the route you will genuinely travel rather than as a picture of a country. The sequence below takes a few minutes and usually changes what you decide to prepare.

1. 1List the places you will actually be, day by dayAccommodation, the main destinations, transfer points, and the routes between them. This list is worth far more than the name of the country, because conditions vary widely inside a single country.
2. 2Mark the points where losing the connection actually hurtsLanding and needing to order a ride, checking in where a document has to be shown, or confirming an identity code delivered to an app. Those are the points to prepare a fallback for — not every moment of the trip.
3. 3Check those points against the map one at a timeZoom to street level for each one instead of judging from a national view. If a point sits at the edge of the shading or inside the lighter band, treat it as uncertain.
4. 4Turn off the newest layer and look againIf the point is still inside a served area with only the older layer showing, the odds of basic usability go up. If it disappears entirely, that location depends on the top layer alone, which is the more fragile case.
5. 5Prepare offline material for the uncertain pointsDownload the map area, save the accommodation address as an image, keep booking documents stored on the device, and note an alternative route. Done once, a gap in coverage becomes a minor inconvenience.

Operator maps versus crowd-sourced maps

Alongside the maps operators publish, there is a second kind assembled from measurements collected by large numbers of handsets. The two have opposite strengths. An operator map covers everywhere the operator serves, evenly, because it is calculated. A crowd-sourced map reflects what real devices actually recorded, but only in the places where people carrying them happened to walk while contributing data.

In practice, reading both gives a better picture than reading either alone. Where the operator map shows service and the crowd-sourced data is dense in the same place, confidence rises. Where the operator map shows service and there is almost no crowd-sourced data, that does not mean there is no signal — it may only mean few people pass through.

Whose map a travel eSIM user should read

This matters and is routinely overlooked. A travel eSIM generally does not operate its own network in the destination country; it runs over partner networks there. The coverage you will actually experience is therefore the coverage of whichever partner network the plan uses, not of the brand that sold you the plan.

The consequence is that before buying you should look at the plan's detail page to see whether it names the networks it uses in that country. If it does, read that network's map along your route. If it does not, ask before you buy. We do not state it on another provider's behalf and we do not guess, because these arrangements change and differ between plans.

• Check whether the plan names the networks it uses in the destination country.
• If it does, read that network's map along your route rather than at national level.
• Turn off the newest layer and look again to see the widest base layer.
• Check the points that cannot fail individually: airport, accommodation, transfer stops.
• Download offline maps for the area while you are still on Wi-Fi.
• Save the accommodation address and booking documents so they open without a connection.
• If the route crosses remote terrain, assume there will be stretches without service.

Where maps are least reliable

Some kinds of terrain and some situations are especially hard for a model to predict. Knowing which ones lets you set expectations and prepare in the right places rather than being surprised on the day.

Mountains, valleys and coastal roads

Radio travels largely in straight lines, so terrain with ridges creates shadows easily. A road climbing a mountain can hold a good signal while it faces a mast and lose it entirely at the next bend. Coastal routes behave similarly, since masts sit on the land side and their reach out over water is limited.

Underground and inside large structures

Metros, tunnels and basement levels depend mainly on equipment installed inside them, which may or may not be present, and a national map does not distinguish between the two cases. Large malls with heavy steel structure and office buildings using coated glass also attenuate signal more than most people expect.

Moments of unusual demand

A map shows coverage, not capacity. At a festival, a concert, or the moment a stadium empties, an area shaded as strongly served can become very slow simply because a great many people are using it at once. That is not a fault in the plan or the handset, and it usually resolves as the crowd disperses.

When you are inside the shading and nothing works

This happens, and it does not always mean the map is wrong. The first job is to separate a location problem from a settings problem. Cycle airplane mode once to force a fresh network search, confirm that the travel profile is the line selected for mobile data, and confirm that data roaming is switched on for that profile.

If all three are in order, step outside or move to a higher floor and try again. If it works immediately in the open air, the building was the problem rather than the plan. If it still fails outdoors, switch to manual network selection and work through the networks your provider lists as supported, one at a time.

Getting real value out of a coverage map

A coverage map is genuinely useful when you ask it the questions it can answer: is this route likely to have continuous service, does this area depend on the newest generation alone, and where should I prepare a fallback. Those are planning questions, which is exactly what a model is good at.

What not to expect is a point-level answer such as how many bars there will be in a fourth-floor room in a particular building. No map answers that, and a small amount of offline preparation makes the answer stop mattering.

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