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What is LIDAR?
LIDAR is an acronym for LIght Detection And Ranging.
It is a measuring system that detects and locates objects on the same principle as radar but uses light from a laser.
In Johnson County's case this was airborne LIDAR that was flown similar to an aerial photography flight. The result is a point dataset with elevation and intensity values for every point. LIDAR is available
for download in the
Digital Data Request (DDR),
when specifiying what data to receive make sure the planimetrics check box is checked,
and you will then have the option to request LIDAR data.
3D Elevation All Returns
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3D Elevation Points
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3D Elevation Bare Earth All Returns
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3D Elevation Bare Earth Points
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How can I use LIDAR in AutoCAD or ArcGIS?
Click here for steps on getting started with LIDAR in AutoCAD and ArcGIS.
When was the Johnson County LIDAR flown?
The Johnson County flight was part of a regional USGS sponsored flight that was flown in the spring of 2006 (roughly March/April 2006)
Map of 2006 regional USGS LIDAR flight
Johnson County LIDAR Specifications
LIDAR data was captured at a nominal post spacing of one 1.4 meters (4.6 feet) and tiled into 5,000 X 5,000 feet tiles. First, Middle and Last returns were delivered in LAS files.
Filtering to bare earth should meet the following standards:
- 90% of artifacts removed.
- 95% of outliers removed.
- 95% of vegetation removed.
- 98% of buildings removed.
Data Accuracy
The bare earth data shall have the following root mean squared errors (RMSE’s) relative to North American Vertical Datum 1988 (NAVD88) and be able to support 2-foot contours (measurements are in metric because it was part of a larger regional USGS project that was also being delivered in UTM):
- Vertical bare earth 18.5 cm (~7.28 inches) Root mean squared error (RMS) @ 95% confidence.
- Vertical bare earth 15.0 cm (~5.91 inches) RMS @ 90% confidence.
- Vertical in vegetation 37.0 cm (~14.57 inches) RMS @ 95% confidence.
- Horizontal 1 meter (~3.28 feet) RMS @ 95% confidence.
What Format is the Data in?
Johnson County received the data in LAS format which is a binary file that is becoming the industry standard. However currently there are some applications that do not read this format. Johnson County choose to redistribute the data in CSV (Comma Separated Value) text files for ease of use. If you are interested in software that will read LAS files reference the "Where can I learn more?" section. If LAS would be beneficial please let us know by contacting the mapper of the day.
Why Do I Receive Multiple Files?
The reason you receive multiple files is because the data has gone through algorithims to determine a classification scheme. Johnson County received data in four classifications: Bare Earth, Low Point, Model Keypoint, and Unclassified.
What types of applications is LIDAR data used for?
The most common use of LiDAR data is to create a Digital Elevation Model (DEM). This DEM can be used for floodplain delineation and mapping, aerial photography orthorectification, breakline generation and contouring. Other uses being investigated are identification of changed areas for building and vegetation updates.
How large is the data?
There are 2 ways to measure the size of the data (disk space required and number of points). A typical 5,000 foot tile has about 2 million points and is about 70 megabytes in size. We are estimating that we have about 980 million points for the entire county (30 gigabytes). Typically it isn't the file size of the data that is the problem, but instead it is the sheer number of points which is overwhelming. This is the reason the data is in a tiled format.
What future plans does Johnson County have for LIDAR?
The current flight was flown as a test to see in what ways the data could be used and how much it would be used by the data partners. Johnson County currently has no plans to repeat the flight similar to our aerial program which is flown every 2 years. We will be evaluating how useful it is and value your feedback. Please let us know if you find it useful by contacting the mapper of the day.
Where can I learn more?
Reference
Software
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