Lava rock looks solid. In a lot of places, it looks flat too. But looks can fool you. If you plan to build a home or a building on lava ground, a topographic survey should be your first call, not your last.
Why Lava Fields Aren’t as “Level” as They Look
Walk across a pāhoehoe field and it can feel smooth and even under your feet. Walk across an ‘a’ā field and you’ll trip on sharp ridges every few steps. Neither surface tells you the real story of the land.
Old lava flows hide small rises, dips, and rock shelves. Some drop a foot or two within just a few feet of ground. You won’t catch this by eye. You won’t catch it by walking the lot either.
A topographic survey maps the true shape of the ground. It shows every rise and dip as a contour line on paper. Builders use this map to pick the best spot for a foundation. Architects use it to plan how a house will sit on the land. Skip this step and you risk a costly surprise once grading starts.
Reading Lava Tubes and Voids Before You Build
Hawai’i Island sits on layers of old lava flows from Kīlauea and Mauna Loa. Some of these flows left tubes behind: long, hollow tunnels formed when the outer layer of lava cooled and hardened while liquid rock kept moving inside. Kazumura Cave, on the east side of the island, is one result. Surveyors have mapped it at more than 40 miles long, making it the longest known lava tube on Earth.
A topographic survey can pick up clues on the surface. A sudden dip, a sunken ring, or a collapsed patch of ground can point to a hollow space below. Surveyors note these features on the map.
But a topographic survey won’t tell you for sure if a tube sits under your lot. It only shows the surface. If a survey turns up a spot that looks off, bring in a geotechnical firm to check what’s underground before you break ground. Think of the topographic survey as the first flag, not the final word.
Why Rugged ‘A’ā and Smooth Pāhoehoe Change How the Fieldwork Gets Done
‘A’ā is rough and broken. Walking on it is slow, and setting up survey gear on jagged rock takes extra care. Pāhoehoe is smoother on top, but it can hide thin shelves and air pockets underneath that give way under weight.
Both types of lava change how long a crew stays on site. A grassy, flat lot might take a crew half a day. A lava lot of the same size can take much longer. Crews need to take more shots per acre to catch every rise, dip, and hidden edge.
This extra time isn’t wasted. It’s what gives you a true map instead of a rough guess. Cutting corners on lava ground almost always shows up later, once construction starts and the numbers don’t match the land.
How Your Topographic Survey Supports Hawai’i County’s Lava Zone Review
The U.S. Geological Survey splits the Island of Hawai’i into nine lava-flow hazard zones. Zone 1 covers ground closest to Kīlauea and Mauna Loa’s active rift zones, where lava has covered the land many times over. Zone 9 sits farthest from that activity and carries the lowest long-term hazard.
Your zone number doesn’t decide what type of survey you need. But it can shape what your architect, engineer, or the county planning office asks to see on your site plan. Setback lines, drainage plans, and structural notes often depend on accurate elevation data pulled straight from your lot.
Bring your topographic survey to design meetings early. Your design team can build a plan around the real shape of the land instead of guessing at it. That saves time later when permits go up for review.
Getting Drainage Right on Porous Volcanic Ground
Soil and lava rock handle water in different ways. Some lava, especially fresh ‘a’ā, soaks up water fast and lets it drop straight down. Pāhoehoe often does the opposite. Water runs across its smooth surface and pools where the ground dips or where old flow channels sit.
This matters more than most buyers expect. A drainage plan built for regular soil can fail on lava ground. Water might pool in a spot no one planned for, or drain away faster than a septic system can handle.
A topographic survey with tight contour lines shows engineers exactly where water will move across your specific lot. That data goes straight into grading and drainage plans, so water ends up where it should, not where it happens to land.
Lava ground rewards early planning. A topographic survey done before design work starts gives everyone, from architects to county reviewers, the same starting point: the real shape of your land.
FAQ
Can a topographic survey detect a lava tube under my property?
Not directly. A topographic survey shows what’s on the surface, including dips or sunken spots that might point to a hollow space below. To know for sure if a tube sits under your lot, you need a separate check from a geotechnical firm.
Does my lot’s lava-flow hazard zone change what survey I need?
No, the zone number doesn’t change the type of survey. It can change what your architect, engineer, or the county asks to see on your site plan, which is where your topographic survey comes in.
Does ‘a’ā vs. pāhoehoe change how long the survey takes or what it costs?
Yes. Rough, broken ‘a’ā ground is slower to walk and survey than smoother pāhoehoe. That extra time on site can affect cost.
Will the survey show where old lava flow boundaries sit on my lot?
No. A topographic survey shows the surface and elevation as they stand today, not the history of separate lava flows. For that, check USGS lava flow maps.
I already have a lava hazard assessment for my lot. Do I still need a topographic survey?
Yes. A hazard assessment tells you the long-term risk level for your area. A topographic survey gives your architect or engineer the exact shape and features of your lot, which the hazard assessment doesn’t cover.
