- We were discussing how to specify coordinate systems in the E57.04 conference call yesterday.
- It is always interesting to see what people know about map projections and coordinate systems.
- The control targets used to locate the scanner position in the real world will have the necessary adjustments to the grid built into them.
During our weekly ASTM E57.04 data interoperability subcommittee conference call yesterday we had quite a discussion on coordinate systems and map projections. It is always interesting to me to see what people know about these related subjects, and often I am surprised, although I shouldn’t be anymore because often people who are very knowledgeable about laser scanning for instance know very little about real world coordinate systems. I was the same way until I went to work for a little company called Blue Marble Geographics, who have one of the leading software packages – the Geographic Calculator. Here is the Wikipedia link on map projections.
The issue that came up on the call was how much metadata we should provide with the file being transferred from one format to another. It looks like we are going to include the coordinate system name and datum with year, but leave it that. An example might be California State Plane Zone 2 NAD 83. Here is why we think that should be sufficient, but if you disagree please let me know.
Since the scanner is generally not located over a known point if you want to use real world coordinates you will have to use targets to determine the scanner position from a resection. The position of the targets will have to be determined based on a control survey, probably using GPS in what is called the RTK – real time kinematic mode as this is by far the most cost effective and accurate.
The control surveyor will, as he/she should be responsible for establishing and certifying the position of the targets. It will be their responsibility to perform the necessary adjustments and calcualtions to insure that the control target coordinates have taken into account such things as scale factor, but this is actually quite simple as the conversion from earth centered coordinates, used in the Global Positioning System, to a local plane coordinate system such as state plane is well established.
Once the scanner location is known the point cloud coordinates can then be converted from an assume Cartesian coordinate system to the desired real world system.