User guide

Open the Project Troll website in your phone's browser. There is nothing to install and no account to create.

1. Answer the cookie question. A box appears the first time you visit. Accept lets the team count which bridges are used. No thanks changes nothing about how the site works.
2. Choose your units. Pick feet / inches or metres near the top of the page. The site remembers your choice.
3. Read the safety reminder. It sits under the units choice: the readings are a guide only, so always check the bridge boards before you pass under a bridge.

The site covers four bridges today: Potter Heigham Old, Beccles Road Old, Beccles Bypass New and Yarmouth Yacht Station (lowest). More are on the way.

The home page gives you every bridge at a glance. Each bridge has a card showing:

• The name of the bridge.
• The headroom, which is the space between the water and the underside of the bridge.
• A coloured badge and the time since the reading was taken, such as "12 min ago".
• A headroom arrow, for example "Headroom decreasing ↓ 4.0 in/hr". It is based on the last hour of readings. Green means headroom is growing, amber means it is shrinking, and grey means it is steady.

The coloured edge of the card matches the badge: green, amber or red.

Tap Refresh to reload every bridge. The page also reloads itself when you come back to it after a couple of minutes.

Tap a card to open that bridge's page. The bridge you opened last moves to the top and is tagged Last viewed. The rest follow in alphabetical order.

A bridge page shows one bridge in detail. Working down the screen:

You can bookmark a bridge page, or send its address to a friend, to come straight back to it.

Enter your boat's air draft (and a name for the boat if you like, otherwise it is called "My boat"), which is its height above the water including the mast, aerials and anything else on top. The box is just under the guidance note on the home page and on every bridge page. Enter feet and inches, or metres if you have chosen metres. It is saved on your phone only.

Each bridge then compares its headroom with your air draft:

• ✖ WILL NOT FIT (solid red, pulsing banner, and a thick red border on the home page card). The headroom is at or below your air draft. Do not attempt to pass.
• ▲ CLOSE FIT (amber, dashed border). You clear the bridge, but by less than your warning margin. Check the bridge board before going through.
• ✔ Fits (green). You have room to spare, and the page says how much.
• ? Can't confirm fit (grey, dotted). The reading is saved or too old to rely on.

The warning margin starts at 1 ft (30 cm). You can change it to 6 in (15 cm) or 2 ft (60 cm) under Warn me when within. If a reading is not Live, the warning also tells you how old it is. Tap Save when you are done, which closes the box. Tap Clear to remove your air draft.

A warning is a guide, not a guarantee. Water levels change, so always check the bridge board and leave yourself a safety margin.

There are two charts on every bridge page: the last 24 hours and the last 7 days. Time runs left to right.

1. Remember which way is up. Higher on the chart means less headroom, so a rising line is the water coming up towards the bridge.
2. Touch the line to see a reading. On a phone, tap or drag your finger along the chart. On a computer, move your mouse over it. A small label shows the height and the date and time. Tap elsewhere to close it.
3. Look for dotted stretches. If no readings arrived for more than 2 hours, the line stops and a dotted line joins the two sides. The dots only connect the gap and do not show what the water did in between. There is no label over a gap, because there is no data there.
4. Check the right-hand end. If the readings stopped more than 2 hours ago, the chart runs on to the present with an empty stretch, so you can see the sensor has gone quiet.

The line is smoothed to make it easier to read. The labels always show the real readings.

Check the badge and the time before you trust a height. The badge tells you how old the reading is.

Two amber messages to know about

• "The newest reading looks out of band, so the previous reading is shown." The sensor sent a value that made no sense, such as a sudden jump. The site shows the last good reading instead. Check the time shown, because that reading is older.
• "When last saved:" in front of the headroom arrow means the arrow comes from saved data, not from the last hour.

Whatever the badge says, the readings are a guide. The bridge board is the final word.

Signal on the Broads comes and goes. The site is built to cope.

• A red banner at the top tells you when you are offline, or when the live data cannot be reached.
• Your last readings stay on screen. Each time a bridge loads, the site saves its latest reading and its charts on your phone.
• Saved readings last 2 hours. After that, the site will not show an old copy. You will see a message and a Try again button instead.
• Everything saved is labelled. Look for the amber Saved reading badge and the time the reading was recorded.

Before you set off: open the bridges you care about while you still have a good connection. The site can only save what it has loaded. A bridge you have never opened will have nothing saved.

Putting Project Troll on your home screen gives you an icon, a bridge arch with a height arrow, that opens the site like an app. You will see an Add to home screen bar on your phone. You can also use the Add to home screen link at the bottom of any page.

On an Android phone (Chrome)

1. Tap Add on the bar and confirm in the box that appears.
2. If no box appears, tap the browser menu (three dots) and choose Install app or Add to Home screen.

On an iPhone (Safari)

Apple does not allow a website to add itself, so you do it from Safari's menu:

1. Tap the Share button (the square with an arrow).
2. Scroll down and tap Add to Home Screen.
3. Tap Add.

If you opened the site from inside another app, such as Facebook or an email app, open it in Safari first. Tap the cross on the bar to hide it for 30 days.

• Analytics is your choice. Google Analytics only runs if you tap Accept. To change your answer, use the Cookie choices link at the bottom of any page.
• The site remembers a few things on your phone only. These are your units, your boat's name, air draft and warning margin, the last bridge you viewed and the saved readings. They are not sent to anyone.
• Buy us a coffee. The coffee button and the small coffee icon on screen come from a separate service, Buy Me a Coffee. They are not covered by the cookie question.

Project Troll is an independent project with no connection to any government body or authority. We would love to hear your suggestions, questions or offers to help sponsor more bridges. Email enquiries@bridgeheight.com and we will reply as quickly as we can.

If you find the site useful, the Buy us a coffee button is a simple way to support the project.

The bridge-height information is provided for personal, recreational and navigational reference. Collecting, copying or reusing it in bulk, or for business, needs written permission. See the Terms of Use page for details.