OxTS Georeferencer outputs pointclouds in an LAZ/LAS format in local coordinates. For many applications, global coordinates are needed. This article will discuss the use of global coordinates with OxTS Georeferencer. Development is being made on Georeferencer to produce pointclouds that make working with global coordinates easier.
The output of pointclouds is given in local coordinates. This means that there is an origin which is at 0,0,0 and all points have coordinates relative to that, given in metres. These measurements are calculated using the INS position in a coordinate datum.
The INS will be outputting navigation data working within a specific coordinate datum. This is used to give the position of points in the pointcloud but the coordinates will be set to relative to an origin which is given 0,0,0 as its coordinates. Therefore, the relative distances between points are correct to a coordinate datum but their coordinates are given explicitly in local coordinates.
There are several choices for the coordinate datum that the INS will be working within. This is selected in NAVconfig when configuring the unit. In Advanced tools>Global Coordinate System you can select a datum and the altitude reference.
There are currently four choices of coordinate datum and we are developing more to be integrated with our firmware. The choices are: WGS84, ITRF2008, ETRS89 and NAD83. The altitude reference can be chosen as Geoid or ellipsoid and can be given an offset.
The origin of the pointcloud is chosen in NAVconfig or in NAVsolve in post-processing. Ensure that your unit has had local coordinates enabled with OxTS support.
Choose an origin from a point on your survey route. This can be done automatically by reading it from a device or you can view the NCOM data for the coordinates. In NAVsolve, a local coordinate origin can be chosen in the Process window, but you will also see the route you have taken and can choose from there. This coordinate must be given with a latitude (deg), longitude (deg), and altitude (m). A rotation can also be applied if you so wish.
If you have a ground reference point that you know the exact global coordinates of, you can use this as the coordinate origin.
When loading your NCOM data into Georeferencer you see where your origin is with respect to your survey route overlaid on a Bing map. This requires the latest version of Georeferencer, 1.4, which has not yet been released (April 2021).
Working with global coordinates
If you are completing an application that requires you to work in global coordinates for a pointcloud survey, the best method is to use a ground reference point and then identify points in the pointcloud with coordinates relative to it.
- Use a GPS locating device to get the exact global coordinates of a reference point in a coordinate datum that you wish to use.
- Input these coordinates as the local coordinate origin for the pointcloud. Points' coordinates in the pointcloud are now in metres relative to the origin you have set.
- Use third party software such as CloudCompare to measure distances and look at coordinates of points. These are in metres and relative to the origin. You may have to convert these into degrees if you require latitude and longitude measurements.
It might be the case that your application would be easiest with global coordinates always in the pointcloud. The following is a guide for converting your local coordinates pointcloud into global coordinates with correct scaling courtesy of our partners Dronezone. Global Mapper software is used but there are other software packages that will allow you to do the same thing.
- Georeferenced pointcloud in LAS/LAZ format
- Local coordinate origin of pointcloud (stored in the LRF file when processing an RD file into NCOM)
- Global Mapper or equivalent software
- Open Global Mapper and select 'Open Data Files'. Select your LAS/LAZ file that you wish to convert.
- You will be presented with a warning and a 'Select projection' dialogue box. A default selection will appear (eg UTM format) which will not be correct.
- Input your correct information. Select 'Orthographic' as the projection type, this is Global Mapper's equivalent of o Cartesian coordinates . Select also WGS84 as Datum and meters for both the planar units and elevation units. Now input the local coordinates origin for the 'Central' longitude and latitude.
- Next you will be presented with many options. Choose the ones relevant to you, default can be fine. Click OK and the data will start to load. You will then get a visual of your data.
- You can represent your data overlaid on satellite imagery. Select File->Download Online Imagery. Then select World Imagery and 'Current Screen Bounds'.
- The data is aligned but will not always be perfect. The accuracy of the Lidar data is 2cm but the accuracy SRTM data of the overlaid map is 3 arc seconds which means approximately 90 meters horizontal and 20 meters vertical absolute accuracy. So a true comparison should only be done with an overlay of similar accuracy to the Lidar Data.
We are committed to giving surveyors all that they need for their LiDAR applications. One element of this is offering the ability to work with global coordinate frames in an efficient and flexible way. There are several developments projects planned to increase this ability.
- More global coordinate datums: The INS will be working within a specific coordinate datum with an altitude reference. We will be expanding our selection of datums to give the greatest flexibility for precise mapping worldwide.
- Global coordinate metadata: OxTS Georeferencer users will have the ability to choose to add global coordinate metadata to their pointclouds. This means that the coordinates will still be in local coordinates by default, but the global coordinate origin and UTM zone for example is encoded into the metadata which allows some third-party pointcloud-viewing software to display the pointcloud in global coordinates.
- ECEF coordinates: OxTS Georeferencer users will have the ability to choose to output their file in ECEF (Earth Centred, Earth Fixed) coordinates instead of global coordinates. These are x, y, z coordinates but in a global datum. ECEF coordinates present the problem of very high x, y and z values and so a loss of resolution can occur, pointcloud viewing software will often accommodate for this by shifting the pointcloud.