I also switched to an equal area projection just to give you a different perspective. Note that NAVD88 geoid grids only cover the U.S. If I create surfaces in EGM08 and NAVD88 equivalent to my original NAD83 zero height and then subtract the NAVD88 surface from the EGM08 surface, I get a map that looks like Figure 2. How different are the orthometric heights that go with NAD83 ( NAVD88) and WGS84 ( EGM08)? Let’s take a look. I did mention that people tend to work in orthometric heights like NAVD88, so what about those? You can get to orthometric heights by applying a geoid model to the ellipsoid heights, but you have to apply the geoid model that’s appropriate to your datum. In a world where we strive for under 10 cm RMSE z accuracy for sea level rise studies, adding a meter shift when converting datums is something you’d better pay attention to. For many mapping purposes, a shift on that order isn’t a big deal, that’s usually not the case vertically.
Click for larger image to read the scale.įor most of the conterminous United States the shift is around -1.0 to -1.5 meters, while in Alaska it’s somewhere around +0.5 meters. WGS84 ellipsoid heights at zero NAD83 (equivalent to WGS84 minus NAD83). Figure 1 shows the height in WGS84 coordinates for the zero elevation in NAD83 coordinates. That program is really meant to do a lot of other cool things related to the velocities of tectonic plates, but I used it for a simple change of coordinates without a time difference.
So, what does the spatial distribution of vertical change for the 3D transform between NAD83 and WGS84 look like? I ran a one-degree grid for the Northwest quadrant of the world through the NOAA/NGS HTDP program to have a look. NAD83 and WGS84 Ellipsoid Height Comparison NAD83 or WGS84) to get those orthometric heights. lidar), somewhere along the line someone applied a geoid model to ellipsoid values (e.g. But, if you’re working with data derived from GPS (e.g. They work with NAVD88 orthometric heights. In the U.S., most people don’t work with either NAD83 or WGS84 in the vertical. It turns out that both NAD83 and WGS84 are 3D coordinate systems and the conversion between them happens along all three axes. However, I work in the vertical a lot and you don’t find much on the web about that difference. If a coordinate shift of somewhere between zero and about two meters doesn’t matter, then you’re good to go. In a horizontal sense, and depending on your application, that’s not unreasonable. If you do much searching on the Internet, you’ll find sites that tell you that “for mapping purposes,” these are the same. In particular, are they the same for mapping purposes or not? One of the things I frequently come across is a confusion regarding WGS84 and NAD83. I do, however, have occasion to work with various horizontal and vertical datums in my day job (if the term datum is new to you, you might want to check out this tutorial on datums). That’s my disclaimer and I’m sticking to it. I work for an agency that has some geodesists, but I don’t even play one on TV.
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