Aerial sensors used to map the contours of the Earth’s surface could help Pinellas County, Fla., officials assess how future property values along the coast will be affected by flood risk and the ever-increasing costs of insuring against it.

The property appraiser’s office meticulously is reviewing data collected across the Tampa Bay area several years ago with airborne lidar sensors, which use lasers to gauge land elevation relative to trees, buildings and other objects.

Examining each home or office building to estimate its elevation over flood levels should make it easier to analyze how insurance costs might affect future land values. It also could be useful for challenging the Federal Emergency Management Agency’s controversial flood maps, which have a big effect on rates.

Although Congress moved to slow premium hikes in the federal flood program for older homes in risky areas, rates still can go up 5 percent to 18 percent a year, meaning some homes could be stuck with unaffordable policies within a few years.

“It’s not an issue for this year’s tax roll, but if the rate continues to increase 15 percent a year, there is going to be a point where they double and then they triple,” Pinellas County Property Appraiser Pam Dubov said.

“At some point in time, houses on the same street that normally would have sold for the same prices may start selling for different prices.”

A number of them might be difficult to sell if a buyer sees an exorbitant flood policy on top of their mortgage. But determining how insurance costs will affect each home’s value has proven to be a puzzle, Dubov says.

In any given neighborhood, a home that is only one-foot lower than neighboring residences could face a premium of several thousand dollars more, an obvious negative for its resale value.

A more hopeful scenario would be that private insurers such as Lloyd’s of London continue to lower rates to undercut the government’s flood program and more of them enter the marketplace, which was the goal of legislation passed this spring in the Florida Legislature.

The harshest of those rate increases were repealed with a bipartisan law that went into effect at the beginning of this month.

The linchpin in debates about accurate risk rating is the height of a home’s first living floor compared with the elevation of the land.

The Federal Emergency Management Agency produces generalized maps that rate an area’s risk based on its proximity to water, threat of storm surge and topography, but the only sure way for a homeowner to gauge the danger is to hire a surveyor to take detailed measurements.

The problem is most of the homes targeted by the government for increased rates don’t have professional elevation certificates, so they often are lumped together in the same risk category as their neighbors.

That’s where lidar comes in.

Several years ago the Tampa Bay region was mapped with Light Detection and Ranging, or lidar, with the goal of modeling storm evacuation scenarios.

The technology, often attached to a helicopter or aircraft, uses lasers to measure the distance between sensors and features on the Earth’s surface to produce detailed topographical data.

The information can be used to estimate a building’s elevation in relation to its surroundings as researchers compare the lidar data with aerial photographs, says Alan Lulloff, science program director for the Association of State Floodplain Managers in Madison, Wis.

The one snag can be assessing the height of the first living floor.

Even if you can determine a building’s height, it can be hard to tell from aerial images whether the bottom floor of a house is raised on stilts, which makes a substantial difference in flood risk.

“Unless you get on the ground and look at them, you can’t often tell if they’ve been elevated or not,” Lulloff said.

Pinellas County’s property appraiser is looking at newer homes that already have up-to-date elevation data and comparing them with the lidar data to see how closely they match up.

“Having the certificates of elevation we have provides the means to validate the technology we’re using,” she said.

Staff is also looking to accurately pinpoint where a building is on its parcel, whether at a high or low point.

It will be some time before they finish examining each property, Dubov said.

Should insurance rates stabilize in the future, she said, her office might not even use the data, but there’s a good likelihood elevation will affect values in the long run.

Having more comprehensive data about flood risk to the county’s homes also could result in lower premiums from the government, which rates communities based on broad safety measures, flood mitigation and property owner education.

Of course, if a homeowner wants to dispute the property appraiser’s valuation, Dubov says they always can order an elevation certificate and mail it to her office. “If they disagree,” she said, “they can prove us wrong.”

©2014 the Tampa Tribune (Tampa, Fla.)