Wednesday, April 18, 2012

About the Maps

On the Mt. Rainier NP photo guide there are quite a few maps for the presentation of the information for sites in the NP associated with each suite of Web pages for the different topics. The user needs to understand the locations for their needs, especially if they're using GPS devices.

All of the locations, the latitude and longitude, are determined using the North American Datum (NAD) of 1927, and not NAD of 1983. All elevations are determined using the National Geodetic Vertical Datum (NGVD) of 1929, often called Mean Sea Level, and not NGVD of 1988. There are reasons for this.

First, it's what I used throughout my career with the USGS (1978-2005) for the location and elevation of gages and other field sites. It's what used for USGS 1:24,000, or 7.5 minute, topographic maps (noted on the bottom center of the map), meaning it's pretty much embedded in my thinking about locations and elevations.

There were and probably still are, discussions within the USGS Water Program to convert all the sites to NAD 1983 and NGVD 1988, and in some cases, the station information has both values in agreement with the cooperator(s) for the station who is using the newer datums.

Second, the information is transferable and convertable to NAD 1983 and NGVD 1988 with any mapping software. You can set the horizontal and vertical datums in the preferences panel except for a few, such as Google Earth. And any GPS unit can switch coordinate datums either through their computer software or application or in the unit itself.

Third, the maps used were initially developed using NAD 1927 and NGVD 1929 which is with Google's map registration settings to work with the information in the XML files. I will eventually add the information for users to read and transfer to their GPS devices or map applications.

I don't expect to convert the new to Google Map API version 3 with the newer coordinates and elevation. I like using the older one for continuity with the XML files and with Google maps. And as noted, the information does convert or translate to the newer coordinates.

Anyway, that's the notes about the maps. You're always welcome to send e-mail with your questions, comments, suggestions or problems.

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