Coastal Towns to Get Free High-Resolution Elevation Data
By Michael Smith, State GIS Manager, Maine Office of GIS (MEGIS) http://megis.maine.gov
Recent news indicates that coastal towns are in for some interesting times. With Portland getting eleven inches of rain in March 2010, southern Maine dealing with floods for the second time in four years, and the sea level creeping up 6 inches in the past seventy years, water is on the minds of a lot of people. How can towns manage and plan to take advantage of their aquatic resources yet avoid the kind of hazards associated with flooding and sea level rise?
One tool that can help is the use of high-quality geospatial data. Current data characterizing the coastline and floodplains for most of Maine's coast are based on USGS topographic maps which are typically twenty to thirty years old. These maps have elevation increments of twenty feet with a spatial accuracy of between ten and forty feet. Obviously, it makes it difficult to model a six-inch sea level rise when your elevation contours are twenty feet apart. Likewise, how does a coastal town simulate a five-foot storm surge, or a river at twelve feet above flood stage with these data? Most towns have updated their computer systems, fire trucks, and plow trucks more frequently than the elevation data currently available to help them plan for flooding or climate change.
Modern technology is available which can precisely measure elevation at intervals of 3 feet, with a vertical accuracy of just six inches. This technology, known as light detection and ranging (lidar) is similar in concept to radar, but uses laser pulses instead of radio wave pulses. These laser pulses are shot out of a machine in the bottom of an airplane at a rate of almost 200,000 pulses per second. The pulses bounce off the ground, or trees, or buildings, and scatter back up to the plane. A sensor in the plane records the time it takes for the pulse to return. That measurement is then converted to a distance measurement. Using very precise measurement tools such as global positioning systems (GPS), the sensor can take into account the position, speed, and movement of the plane to calibrate the distance measurement precisely, turning that into a measurement of the elevation where the pulse hit.
(Above): The laser pulses bounce off the ground (or trees or buildings) and back up to the plane, where the distance is precisely measured.
Since the pulses can hit bare ground, trees, or buildings, there are multiple “images” that can be teased out from these pulses – bare ground topography, forest canopy height, or urban terrain models for example.
The resulting data create a very precise model of the elevation and topography of an area. These data can then be used by engineers to model flooding, sea level rise, and storm surges. These same data can be used in a variety of other ways: to map wildlife habitat, predict erosion, model suitability of potential wind energy sites, choose location of cell towers or wireless broadband equipment, and predict forest types.
|A lidar data file shown in PointVue
|A 1-meter elevation model from lidar, showing slope
Prouts Neck, ME
Lidar intensity data - Scarborough, ME
As part of the American Recovery and Reinvestment Act (ARRA) of 2009, almost nine million dollars was included to collect aerial photos and lidar data for 'priority areas' from the US Geological Survey (USGS). The priority areas were primarily coastal areas for which lidar data did not already exist. MEGIS worked cooperatively with the other New England states and New York to develop a joint proposal which included the entire coastal strip from New York City to Cobscook Bay. The data we proposed to collect included lidar at a 6.5-foot point spacing, with a 6.5-foot digital elevation model, and other lidar products. Of these, the most popular will be the digital elevation model which can be widely used to help map floodplains and model coastline changes. Compare that to Maine's best currently-available data, which has a very coarse 35-foot resolution. We will also take advantage of lidar that was collected by USGS and the Federal Emergency Management Agency (FEMA).
Data collection for the project is expected to occur during the Autumn of 2010, with data processing and delivery by mid-July 2011. As with most other ARRA-funded projects, this one is fast-tracked in the hopes the money will stimulate the nation's economy. When that time comes, all the towns in the project area can expect free access to the products, and use them to better plan for their future. The area to be collected in this project is shown as ARRA Data Collection in the graphic below.
This project was supported by the Maine Library of Geographic Information (GeoLibrary) http://www.maine.gov/geolib .