Terrain Point Cloud Extraction from High Resolution Optical Images

If you've been to ERDAS Labs then you probably know that we are developing a new automatic terrain extraction solution. One of the unique things about the project is that it has given us the opportunity to think carefully about how we persist terrain data, and last week I had a chance to discuss this at the ISPRS Workshop in Hannover, Germany. I don't have a recording of the presentation, but here are some of the details:

1) Digital imagery is achieving increasingly high resolutions. We are now at a stage where airborne sensors can achieve higher than 5 centimeter pixel resolution.

5cm Resolution ADS80 Imagery

2) Many softcopy auto-correlation systems (XYZ terrain point matching system) were initially developed upwards of a decade ago, and were not designed to take advantage of high resolution sensors. Our own LPS ATE module was originally released in 2001 as "OrthoBase Pro" (timeline here). One of the features of some more modern systems is the ability to attempt correlation on every pixel - which can yield a very large volume of data.

117 Million Auto-Correlated Terrain Points

Detailed View: Terrain Points on Individual Boats

3) TINs and Grids, the traditional formats for persisting terrain data in softcopy photogrammetry and GIS, may not be optimal for high resolution terrain data: hundreds of millions of points at a high density. Both have pro's and cons, which Gene Roe has outlined here. Grids can be redundant (particularly for flat regions) and while TINs are very flexible in this regard, they have no standard format. Each vendor has their own implementation, making data translation and transportability a challenge - not to mention long-term storage.

4) The LAS format, while designed for use with LIDAR sensors, may be a viable alternative to TINs and Grids for autocorrelated terrain data. Why? There are a few different reasons:

• LAS is an ASPRS-administered standard and has a high adoption rate among geospatial software vendors.
• The LAS 1.2 specification supports attribution, for example the ability to encode an RGB value for each terrain point. While it isn't commonly used within the LIDAR community, it is very useful for auto-correlated terrain. This allows RGB-encoded terrain to be used for applications such as visualization. Capabilities such as this are not possible with the traditional TIN/Grid approach.
• When correlating on every image pixel, terrain data can be very dense. A compelling research area involves applying LIDAR classification and filtering techniques to autocorrelated terrain data.
  

Point Cloud with Color Attributes (CIR, Red, Green: ADS80 Imagery)

Detailed View: Point Cloud with Color Attributes (CIR, Red, Green: ADS80 Imagery)

The images above show color attribute encoding for an LAS 1.2 point cloud that was processed from stereo ADS80 imagery. The bottom image is a zoomed in perspective view showing a lot of detail: solar panels on the roof, cars, and a feeling of depth in the empty pool. These images show the point cloud rendered as a TIN within the FugroViewer. As you can see, the terrain representation is quite different from a traditional TIN or grid.

Leica XPro: A First Look...

Last week I had the opportunity to learn more about Leica XPro, the new ground processing software for the ADS40/ADS80 airborne sensor. You may have seen the press release from ISPRS last summer announcing the partnership between Leica Geosystems and North West Geomatics Ltd.

Some background information: GPro has been the post-processing software for the ADS40 pushbroom sensor since it's introduction to the market several years back. Coupled with ORIMA, it produces various image products ranging from raw images to triangulated stereo pairs and digital orthophotos.

Leica XPro is the successor to GPro, and is a completely new software package that offers some innovative tools for streamlining the ADS workflow.

Here are a few of the benefits that impressed me:

- There's a new viewer that is blazing fast. Since ADS sensors collect long "pixel carpet" strips of imagery, the file sizes can be fairly large (which can be a big advantage when it comes time for image mosaicking). This allows for a quick quality control check right after the data is downloaded. The viewer allows RAW images to be previewed prior to georeferencing and also supports virtual on-the-fly viewing of L1 (georeferenced) imagery. The viewer also has the ability to apply radiometric corrections on the L1's. Here's what the viewer looks like with an ADS80 FCIR image loaded:


- There's been a complete overhaul of the aerial triangulation methodology. Like GPro, ORIMA-M is the backbone of the triangulation system, however XPro has created a "grey-box" version of it. ORIMA's CAP-A bundle adjustment software is still embedded with the system, but the user interface has been completely updated. There is a completely new system for point measurement (featuring the new viewer technology), and improvements have been made in autocorrelating points across images. You can also load a DSM of the project area (GTOPO30 comes with the software, or you can use your own), as displayed below:

- Also new for the triangulation system is an innovative color coded quality "heat map" of sorts, which allows for an interactive analysis and refinement of weak areas (e.g. point review, new automatic point measurement runs, etcetera). The number of APM points required for a solution has also been dramatically reduced.

Once the imagery is triangulated, XPro can also perform distributed orthorectification processing. Like GPro, the system is also uses CONDOR.

Here are some images of varying resolutions over the same area showing what you can expect out of the ADS80. Feel free to click on the images below to view the full screen captures.

Sensor Spotlight: Leica Geosystems ADS80 Airborne Digital Sensor

I’ve touched on ADS40 sensor technology in a few different posts, but the focus of today is the new ADS80 sensor. The ADS80 is a pushbroom airborne sensor that was formally announced and highlighted at the ISPRS conference this past summer in Beijing.

See here for an interesting discussion on the transition to from analogue to digital processing as well as pushbroom sensors. The new sensor represents a solid advancement, and arguably delivers the best quality imagery of any of the commercial large-format airborne sensors.

But what is the difference between the ADS80 and the previous version, the ADS40? This post will cover the differences and explore some of the specific technical improvements.
Firstly, there are several overall design improvements. There is a new design for the data channel with overall data throughput increasing from 65 MB/s to 130MB/s. The fastest cycle time has increased from 800Hz to 1000Hz (this allows for faster flying speeds than previously possible), and there are data compression options for 10 bit, 12 bit, as well as the raw data.

The ADS80 also features a new design for the Control Unit (called CU80). The new Control Unit is smaller and contains an integrated slow for two Mass Memory units. Here what the new CU80 looks like:

The new system also introduces a new solid state Mass Memory unit (MM80). This size is smaller and weights only 2.5 kg, and has a few different options for data storage modes: single volume, joined volume, and in-flight backup. The joined volume of the two MM offers the greatest data throughput as well as the largest storage capacity, which is ideal for large-area collection missions.

For direct georeferencing applications, IPAS comes embedded in the control unit as well. This is critical for image collection missions in remote areas where ground control may not be possible: this is important in applications such as disaster mapping, remote area mapping (e.g. certain pipeline mapping applications) as well as surveillance operations.

Overall, the system weight has been reduced by 26 kg! It also contains new periphery equipment, including a new GPS/GLONASS Antenna.

Lastly, what does the imagery look like? In short, it looks fantastic. Here’s a sample of imagery collected at 5cm GSD over Lucern, Switzerland earlier this year (click on the image for a larger view).
More information, including both a product brochure and data sheet, is available from the Leica Geosystems website. Also note that new a new software package for ground processing, called XPro, will also be released quite soon.

Special thanks to Ruediger Wagner, ADS Product Manager at Leica Geosystems, for providing details on the new sensor.