How to Map an Atoll

Posted by mapmeld on 8 April 2013 in English (English)

My GPS trace

Me and my OSM Shirt

What did you map?

I collected a GPS trace between islands of Majuro Atoll, Marshall Islands. Majuro is a long string of islands at most a few city blocks wide, and sometimes smaller than a house. During low tide, you can walk across old, solidified reef from the main island out to Ejit, Kemman, and other tiny populated islands.

What are atolls?

Great question! Charles Darwin went on a ship called The Beagle hoping to sort this out. His measurements convinced the geologic community that atolls come from a coral reef around a volcanic island. After millions of years of inactivity the volcano sinks into the lagoon, leaving the ring of coral islands behind.

Today you can just look at this GIF:

animated atoll formation

How to Prepare

  • Look at OpenStreetMap. Figure out where you are and where you are going.
  • Talk to a local person about where you plan to map. Best case: they come with you!
  • My sneakers and flip-flops were not good 'reef shoes'. I met a guy who'd been living on small islands for years, and he wore a cross between crocs and Batman. I believe they are called fisherman's sandals. Get these before you go to the atoll.
  • If you're using your phone's GPS, try out an app such as My Tracks (Android) in airplane mode before you go the atoll.
  • Visit a site such as to see when the tide is lowest. To maximize your time, arrive before low tide. Find out when high tide is, so you can finish your trip before it comes. Don't trust the timetable: see if others are crossing.
  • Don't cross when tsunami waves or bad weather are expected. Even a small tsunami causes a fast current between islands.

What to Bring

  • Bottled water, crackers, first-aid kit, and sunscreen in backpack
  • Backup clothes (you will get wet)
  • Backup footwear (in case you lose a shoe)
  • Camera and GPS for mapping
  • Local phone (separate from GPS, so your battery doesn't run out)

My Experience

Where to Go

I went mapping on Majuro Atoll about three times. Each time I started from the Djarrit / Rita end of the main island and walked west, once as far as the island marked Bwokwmeej. The tide came in while I was walking, and I ended up catching a boat ride back to shore. A smarter traveler would probably stop at Amiel.

The Taiwanese ambassador said that you could walk from the Laura end of the main island and go north, to Ajokwala. My boss explained that this is possible only during low tide, at certain times of the year, and you can't cross back to the main island on the same day.

One day I went mapping on Arno Atoll, which is several miles to the east. I didn't know the area well, and there was no cell phone service, so I only mapped on land.

Water Depth

I am short. Your experience may vary.

Water is counter-intuitively most shallow on the ocean side of the atoll. You will see people walking in a U-shape away from one island, toward the ocean, and then to the next island. See my GPS trace at the start of the post for an example.

The principal at Ejit warned me that small sharks are a real danger once the tide comes in, possibly even in knee-deep water. I saw several small fish, brittle stars, and a couple of fish which were about the size of my hand.

Once the tide starts coming in, it is fast. When water is nearly waist-deep, the current can easily make you lose your footing. There is a ferry at Ejit, so you can end your walking trip there if the tide is getting too deep.

Shoes and Clothes

I wore a T-shirt, knee-length shorts, and flip-flops. In the photo you can see I wore my State of the Map US 2012 "Map The World" T-shirt. Recommend.

After my first trip, a friend picked me up in his car. I brought a towel to keep the seat dry.

My flip-flops never protected my feet properly. You want to avoid having coral scraping at the toes and sides of your foot. You want the back of the foot to have a tight fit. You can't have closed shoes because water and sand is flowing all of the time. Fishermen's shoes appear to be the right thing to do.

A flip-flop slipped off between Amiel and Kemman islands. Fortunately it floated and I caught up to it. On another trip it broke and nearly stranded me. Walking barefoot on coral-rock is painful. I wish I brought sneakers or a backup pair of sandals.

Meeting Strangers

In the Marshall Islands, people say yokwe to greet each other. It is pronounced a little closer to yawk-way. Visitors do not typically walk very far between islands. People will be surprised to see you.

I've read The Most Dangerous Game. I was cautious at first.

Children on Ejit were friendly and curious. They have met many Americans through the Dartmouth volunteer program.

Beyond Ejit, you will see only a few people. On Amiel I felt completely alone, and was a little disappointed to see a sandal print in the sand. Islanders live on the lagoon side there. Islands to the west are so remote, that there is often only one house.

Respect for Property and Culture

Land is scarce in the Marshall Islands, so land rights are important. You cannot just camp on an island. You cannot just buy an island. If you are interested in renting an island or having a beach day, ask the RRE hotel about Eneko.

There is a small shipwreck and graveyard on the lagoon side of the island labeled 'Pegerian'. I saw them many times while traveling by boat to Ejit, but I did not pry.

You are not National Geographic. Avoid photographing naked people.

Mapping your path

If you drop GPX, KML, GeoJSON, and CSV files in, you'll get an animated timeline-map. Here's mine.

The tiniest island

Location: Rita, Majuro, 96960, Marshall Islands

Comment from skorasaurus on 9 April 2013 at 18:03

This is an awesome diary entry: Great detail, illustrations, and context describing your experience. Thanks for writing. One of the best that I've read in some time.

Mapping an atoll doesn't happen very often. How did you have the opportunity to map there ?

Comment from mapmeld on 9 April 2013 at 18:41

I was training teachers to help start a One Laptop per Child program in the Marshall Islands:

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