LiDAR is changing how we model our world in 3D, bringing with it new data transformation challenges and huge data volumes. It also highlights some old and new challenges with coordinate systems. These largely boil down to getting your data into a form where it can be overlaid with other data and where useful properties are maintained (e.g. water flowing downhill).

How are coordinate systems used with LiDAR?

LiDAR data may be captured in a local coordinate system (for smaller projects), in geographic coordinates (latitude, longitude, ellipsoid height), or geocentric coordinates (also called Earth Centered Earth Fixed). Geocentric coordinates are Cartesian coordinates whose x, y, and z ordinates capture the distance from the earth’s center of mass along the interesting axes (based on the International Reference Pole and Meridian; roughly the north pole and Greenwich). This system is very appealing for LiDAR as the x, y, and z ordinates all use the same units (e.g., meters) and can be used seamlessly anywhere in the world.

What are the transformation challenges related to LiDAR and coordinate systems?

The first requirement is transforming your data into a coordinate system suitable for combining with other data. For example, geographic or geocentric data may need to be converted into a projected coordinate system such as a US state plane. Similarly, data in a local coordinate system may need to be georeferenced and converted into a geographic or projected system.

Second, you may also need to convert the heights into a useful form. Typically, geographic and projected coordinate systems use ellipsoid heights. Ellipsoid heights measure the perpendicular distance to an ellipsoid (flattened sphere) which approximates the earth’s shape, and are easy to measure using GPS anywhere in the world. They aren’t sea-level heights, though, and so they are counter-intuitive near the coast and don’t capture the direction water flows (down) over large areas. As discussed in detail in an earlier post, you can use geoid-based techniques to convert between ellipsoid and orthometric heights to restore these properties.

How is coordinate system metadata stored with LiDAR data?

The most common LiDAR format, LAS, stores coordinate systems using GeoTIFF keys. However, there are a few details yet to work out around geocentric and vertical coordinate systems. Ongoing standardization on this front will be good for interoperability.

The ASTM E57 3D file format, which aims to store 3D and LiDAR data more generally than the LAS format, is under development and its authors are thinking about these issues as well. An early suggestion was to store coordinate systems and datums as names, which I fear would be challenging from an interoperability perspective, but a more recent summary states that OGC specifications will be used instead. I haven’t looked at the specification to see the details (e.g. “Is it OGC Well Known Text?”, “Can I use EPSG references?”, etc) but I will certainly follow the outcome with interest.

Transforming your data (LiDAR, raster, vector, …) into the right coordinate system allows you to combine it with other data and take advantage of important properties like heights. If you’re interested in learning more about the transformation of LiDAR data, join us for a free webinar on April 28 to see how FME can help. What data transformation challenges are you facing today?

3D About Data Coordinate Systems LIDAR OGC Transformation

Paul Nalos

Comments

4 Responses to “LiDAR and Coordinate Systems: Journey to the Center of the Earth”

  1. I have proposed that the next LAS specification be augmented to include (optional) support for OGC WKT. You can find my proposal at http://liblas.org/development/wkt.html

  2. Paul Nalos says:

    Hi Howard,

    Thanks for your note about LAS and OGC WKT. One of the challenges with WKT coordinate system descriptions is that they are underspecified (i.e., come in different flavors as noted in your proposal). I talked about this a bit in an earlier post. This makes them more difficult to read, match to existing known coordinate systems, and write in a way that other software is likely to be able to read. That said, if the WKT references EPSG definitions, these concerns become less relevant. Details aside, I find it encouraging that both LAS and ASTM E57 are trending towards similar solutions for representing coordinate systems.

  3. Great Post! I think scattered light and its return values when encountering surfaces of different reflective quality presents huge challenges to LiDAR users.

  4. Paul Nalos says:

    @Data Transformation: Thank you. I hadn’t previously thought about the issues around LiDAR reflections and different materials — our focus so far has been transformation and translation of data after it has been turned into a point cloud (e.g., a LAS file) — but these are certainly interesting things to consider.

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