USGS and Google Maps

Well, I finally got the NP boundary onto Google maps (it's on the terrain version in some areas of the NP). It's currently on the access Web page. But in reviewing that Web page I discovered some problems with the locations of the markers. I have updated them on this Web page but in doing so realized a small problem.

For one when you blow up the scale of the Google map you will encounter some markers appear to be misplaced or mislocated on the map. Ok, there are two reasons for this. First, user error. I did miss identify the latitude and longitude on some and I'm reviewing all the markers for location. To do this I'm using National Geographic TOPO software which are scanned USGS topographic maps.

And here's where some problems arise. First, their scanning and location algorithm isn't 100% accurate. That's expected as you can only get so good. I will say NG's is the best topographic software, but it doesn't mean the location algorithm is perfectly accurate. It's quite good but leads to the other problem.

The USGS has always used the North American Datum of 1927 (NAD 1927) for horizontal control of location and maps. It's the standard for all their topographic maps. However, a new standard, the North American Datum of 1983 (NAD 83) was developed and in use for many applications. The differences are small but significant for accurate locations on maps.

This is why markers may appear off, while I use NAD27 consistently throughout my photo guide, Google and others will use NAD83. If you're using the coordinates from my maps or Web pages, just switch your application back to NAD27 and you'll find the locations are accurate as reasonable possible from 1:24,000 USGS maps.

The second issue and sometimes problem is elevation. Again I use the USGS standard, National Geodetic Vertical Datum of 1929 (NGVD29), formerly Mean Sea Level (MSL). The newer standard, North American Vertical Datum of 1988 (NAVD88), uses a different algorithm of the earth shape.

Unfortunately, unlike horizontal datum differences, vertical datum differences between the two are really different, sometimes in the 10's of feet. If you're using a GPS, you're likely using NAD83 and NAVD88, so you will need to translate if you use a USGS topographic map or the maps on this photo guide.

You see, I'm a long time user of NAD27 and NGVD29 and have no real interest or intent to swich. This is because I still use USGS topographic maps which uses them. And after 28 years in the USGS, I'm used to it and comfortable with it, long before topographic software was available and long before NAD83 and NAVD88 was accepted by the USGS (still isn't completely).

Anyway, that's the entry to date. Lots more work ahead adding the NP boundary to all the 25+ maps in the photo guide and checking the location of all the markers with each map. But you can know what standard I use and switch if you're using the newer one and want to use the locations for your use.