An overview of the city’s 2015 aerial update
By David Buehler
City of Marshfield GIS Coordinator
Most of you probably did not even notice as a few planes flew overhead back in April. They just looked like any other airplanes in the sky, that is until you noticed that the planes flew a pattern of straight rows back and forth over Marshfield like a farmer planting a crop. These flights were part of the city of Marshfield’s 2015 aerial update project. The project is well underway, and the finished product will be delivered by the end of December 2015.
One might ask, “I have Google or Bing maps on my phone. It has imagery. I can see my house, and my phone knows where I am because it has a GPS in it. Why are we doing this aerial update project?” To answer this question, you need only look at the resolution of the photography.
Both Google and Bing do purchase their own aerial photography products from various entities. The difference is the usage and resolution. The large commercial entities use it primarily as base map for giving directions or finding an address.
Have you ever wanted to zoom in closer to figure out what something is but were unable to? This is because they have a resolution issue. Resolution in this case means how big of an area each pixel takes up in real world space.
Most Google and Bing imagery is meter resolution. Each pixel is roughly 3 feet by 3 feet, which makes features blurry and grainy as you zoom in closer. This is done to make it cost effective to cover the entire country and much of the world.
The city of Marshfield uses a higher resolution aerial photo for its work. The pixels are 6 inches by 6 inches. In addition to a much clearer picture, the ability to zoom in further to extract detailed information and features in leaf-off conditions is crucial for use in engineering projects, planning for a new development, or helping you get those permits for the new shed or garage addition.
This purchase of a new aerial photograph is nothing new. The city of Marshfield has purchased aerial photographs as far back as 1957 and approximately every five years since. The last update project was done in 2008, making this flight two years overdue.
Over the years technology has improved. The early flights were black and white photos and were produced using actual film. With today’s use of GPS technology and digital cameras, the quality of the photo and the resolution has increased exponentially. What makes the 2015 flight different than previous flights is that in conjunction with the better GPS and digital cameras, a newer technology called Light Detection and Ranging (LiDAR) was used.
The LiDAR portion included in this year’s package is produced by a sensor that emits pulses of light at the Earth’s surface. After striking an object on the ground, the light is reflected back to the aircraft. The reflection could be from a tree, a car, a roof, road surface, grass, or driveway that light encounters. The onboard equipment then measures the time it takes a particular pulse of light to return. In post processing the software pulls the data from the sensor and computes additional information for each point in a pulse, such as how long it took to return, a return value, statics about margin of error, and calculated elevation.
Coupling these millions of points of data with a high accuracy GPS and ground control produces an accurate surface model of Marshfield. This LiDAR surface model is then married with the digital photograph of the Marshfield area to produce the highly accurate georeferenced aerial photograph, meaning the photograph is digitally represented exactly where it is on the planet.
These two portions of the aerial update are then used the make the final product of this project called planimetrics. Planimetrics are digital representations of features from the photograph. Think of it as a digital form of tracing paper that gives the features some intelligence because each object knows what it is, where it is in space, and knows its elevation.
Every department of the city uses the photo and the planimetrics to carry out daily operations. Some of the heavier users in City Hall are the engineering division, the planning and economic development department, and emergency services. Many citizens of Marshfield use this photograph as well when using the city’s interactive map viewer. These products save a lot time, which translates to real dollars for the taxpayers of Marshfield.
Some of the uses are field work and data collection of infrastructure, planning for variety of events like Dairyfest or a natural disaster, providing a discussion piece and context with which to drive economic development, planning for new development, permitting, and new construction projects.
A unique aspect of this project is newly created partnerships that helped fund this update. In the past the city of Marshfield partnered with Marshfield Utilities to purchase new aerial photography. Often these flights would be on an arbitrary year like 2008. In 2010, a statewide program of acquisition of orthophotography — aerial photography — was put together by a group called the Wisconsin Regional Orthophotography Consortium (WROC). The consortium encourages governmental bodies to cooperate to benefit from economies of scale and cost share the acquisition expenses of aerial photography products.
This year’s project, though put off two years due to budget constraints, allowed us to partner with Marshfield Utilities, Wood County, the city of Wisconsin Rapids, and the Wisconsin Rapids Water Works and Lighting Commission through the WROC program. This also aligns Marshfield’s schedule with those partners, and the program allows us to access the best prices on these types of services in the future.
This allowed the city of Marshfield to purchase the same quality of products as in previous years for a substantially lower price, while at the same time allowing our partners to obtain our highly detailed level of products at a reduced rate. Putting this in perspective, by cooperating with our partners and building that economy of scale, the total project cost was reduced by approximately 30 percent.
If you would like to know more about where we are in the process or would like to know more about this topic, contact David Buehler, Marshfield GIS Coordinator, at 715-486-2076, email email@example.com, or stop by City Hall at 630 S. Central Ave., Suite 602, sixth floor.