Aggregated ERDAS Posts

As you may have noticed from the links on the right, there are several ERDAS-oriented blogs out there. I've been experimenting with Yahoo Pipes - which has impressive functionality - allowing you to combine and manipulate multiple feeds. I embedded the feed results in a web-page here, so please feel free to check it out. It is also located in the links section on the right: e-planet.

Mapping Accuracy

The focus of today’s post is the importance of mapping accuracy and how it relates to geospatial data. Personally I think accuracy is one of the most difficult and misunderstood aspects of our industry. How many people out there in the geospatial industry have used data (vectors, orthos, or other spatial data) where you have no definite information about the accuracy of the data? How often can people confidently state that their data conforms to an accuracy standard? Furthermore, how many people have detailed information on the lineage of their data? For example, if you have a “roads” vector layer – where did it come from? Was it collected from high-accuracy aerial photography (where you know the triangulation results) in a stereo feature extraction system? Or was it digitized in 2D from a scanned map of indeterminate origins? These are critical questions for applications that require accurate data for making business decisions. The importance of accuracy is also application-specific. For example, in the USA the DOT’s (Departments of Transportation) require high-accuracy vector mapping data. If a road is built incorrectly, it is extremely costly to correct. Conversely, for an application such as a tourist map of a downtown area, often the relative accuracy is the only important factor. The precise coordinates aren’t so important, as long as everything lines up correctly and the person reading the map understands where they need to go.

For high-accuracy geospatial projects, customers (the organization purchasing the data) specify the accuracy requirements. Right now there are several standards for terrain, orthophoto, and mapping data. The USGS National Geospatial Program Standards are a good example. Other standards such as Federal Geospatial Data Committee standards can be found on the ASPRS Standards page. For a detailed look at accuracy standards, check out the Idaho Geospatial Committee's document for review on Aerial Mapping and Orthophoto Standards. It discusses accuracy in terms of the end-to-end photogrammetric workflow, from planning through final product generation.

I would also like to highlight Fugro EarthData’s recent Spring 2008 newsletter. One of the pieces, “The Call for a New Lidar Accuracy Reporting Framework”, features a discussion by industry experts Lewis Graham (GeoCue Corporation) and Karen Schuckman (Penn State University). This is a good read and I would recommend it for anyone interested in the topic of terrain accuracy standards. One of the key points is that many customers still define their project requirements in the form of contours: 2D cartographic representations of 3D data. Other topics explore horizontal accuracy, accuracy reporting standards, spatial variability of error, lidar processing errors, and more.

Sensor Spotlight: WorldView-1

WorldView-1 is a satellite sensor that was launched in September 2007. Built for DigitalGlobe by another Colorado-based company called Ball Aerospace & Technologies Corp, the satellite is in orbit at an altitude of 496 kilometers. There's a great gallery of the construction of the satellite here.

There is a specification sheet on the DG website that highlights the features and benefits of the sensor. WorldView-1 is a panchromatic sensor that features a 0.5 meters GSD at nadir and 0.59 meters at 25 degrees off-nadir. This makes it one of the highest resolution commercial remote sensing satellites on the market today. See a gallery of imagery here.

Imagery can be purchased at various levels of processing. These include:

Basic: the imagery is provided with a camera model and radiometric/internal geometry distortions are removed.
Standard: georeferenced imagery (but not orthorectified).
Orthorectified: 0.5 meter panchromatic orthos.
Basic Stereo Pair: the imagery has orientation information that allows for 3D content generation in a photogrammetric system.

These products give consumers a lot of freedom in determining the best solution for them. For example, if you want complete control over all the photogrammetric processing, then the Basic product will be the best choice. For GIS users that want an imagery backdrop for their application, the orthorectified product is a good option. For users that don't have any ground control (for triangulation) but still want to extract 3D information in a photogrammetric system, the Basic Stereo Pair is a good solution. This option is also good for remote areas where collecting ground control is difficult, expensive or dangerous.

We added support for WorldView-1 data in the LPS 9.2 release, which will support all the products mentioned above. Specifically, you could use LPS to triangulate the imagery, adjust the radiometry, extract and edit terrain, extract 3D features, create orthophotos, and create final image mosaics.

Lastly, for more information on satellite photogrammetry check out this article in the Earth Imaging Journal. It is written with IKONOS imagery as the example but is applicable for Worldview-1 data processing as well.

Angkor Wat in 3D

Today I'd like to highlight a project from the ETH's Institute of Geodesy and Photogrammetry in Zurich, Switzerland.

The project, "Reality-based 3D modeling of the Angkorian temples using aerial images", was performed in 2006 and focused on the start to finish processing workflow for 3D modeling of the temple complex near Siam Reap, Cambodia. The project area was captured by a Leica RC20 camera in 1997 and provided to ETH by the Japan International Cooperation Agency. The project website proceeds to outline challenges (e.g. lack of ground control points within the project area) and processing methods (including LPS) employed during the creation of the 3D models of the temples.

The downloads section of the project page includes an excellent paper outlining the entire process.

If you haven't been to Angkor Wat, it is an amazing place. Here are a few photos I took there a couple of years ago:

More China Earthquake Imagery

Here's some good before and after satellite imagery of an earthquake area - although no mention of which satellite captured it...

LPS Terrain Editor: Terrain Editing Tip

For today's post I thought I would share a tip for a method of quickly editing TIN terrain datasets in LPS. In the LPS 9.2 Terrain Editor, we added a new "Terrain Following Cursor" mode. This was added as an icon in the Terrain Editing panel:Here's how it looks in the panel:
The terrain following cursor snaps the floating cursor to the height of the terrain for whatever XY location you move the cursor to. It operates in either an "on" or "off" mode, so you can easily roam around your imagery and the Z value (height) of the floating cursor will adjust automatically based on the terrain.

So why is this important?

Because it allows you to remove erroneous elevation spikes very quickly! When used in conjunction with the "Delete Tool" you can quickly identify points that are off the ground and delete them, or alternatively adjust them to their correct height. You can use the Terrain Following Cursor mode with any edit tool - here it is while using the delete tool:This means you can use it to easily locate problem areas such as the one below. See how the post is off the ground - the elevation of the point is 551 meters, when it should be at 569 meters.
Here's what the surface looks like after removing the post (a single-mouse click). This can be quite useful as a quality control tool - perhaps after completing area editing operations. After deleting posts, the floating cursor snaps back down to the surface. This allows you to quickly maneuver through a scene removing (or adjusting) TIN mass points. On a final note, you can even map Terrain Following Cursor mode as an event on your input device (e.g. middle-mouse button, or a Topomouse button).

China Earthquake Aerial Photography

There has been extensive media coverage of the recent earthquake in China. This was a major event: a magnitude 7.9 quake that did some serious damage. I noticed yesterday that Newsweek released created an interactive page showing some aerial shots of the damage. The images of Yingxiu and Dujiangyan show how the areas around the epicenter were absolutely flattened.

Here is a link to the USGS details page on the event. The "Maps" tab has several types of maps available, including a KML file of recent earthquake activity (in Google Earth below). It will be interesting to see if other geospatial datasets are made available of the disaster area in time - I would think there is a lot of aerial reconnaissance going on...