Saturday, September 12, 2015

Horizontal Datum

I've been reviewing the map with the current news, information and access and discovered when you zoom in on specific places, the points I have for Google Map, which uses NAD83 latitude and longitude, plot differently on Google Map than for the map applications I use for locating places.

This is because Google uses WGS84 and not NAD83. This is due to the fact Google needs a consistent world horizontal datum and not one specific to North America which I've used throughout my career with the USGS and with the maps for the Mt Rainier photo guide.

The difference can be small, almost insigificant, or obviously close but not right. So, I'll be adjusting the map for the news to WGS84 and checking the locations with Google Map. If this works to correctly plot the locations, then I'll work through all the maps to correct them to WGS84, but that will take some time.

In the meantime the difference is small enough to still identify what the point is and where it is in Mt Rainier NP. But for the information on the maps for elevation, they will still be NGVD 1927 and not NAVD 1988. The USGS still uses NGVD 1927 for a lot of application, reports and uses, so that's still the standard I'll use on the maps with the photo guide.

If you have problems with the elevation, all computer and mobile device map applications have toggles to switch between the two vertical datums, so it's easy to translate between them for your needs or work. As they say, if anything changes, I'll keep you posted.

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