Saturday, July 20, 2013

Do you know the Datum of your Latitude and Longitude?

Last month I had the opportunity to present GeoFlow, now called Power Map, to the GIS team at my employer. Less than one minute into the live demo, I heard something like "That looks like a Mercator projection expecting a WGS84 Datum." Inside my head I was thinking "a what?"

I was not familiar with either map projections or datums. As datums are important when using latitude and longitude data, I'd like to share some of what I've learned about the topic since that presentation. The concept of datums can be quite large, so I'll just focus on what matters most to those of us using tools like Power Map and Power View. Those tools use Bing Maps for their geocoding. The primary point to make is that Bing Maps expects a WGS84 datum.

If your latitude and longitude coordinates do not correspond to the WGS84 datum, then your geographic points might not display in the proper location in Bing Maps. Let's first cover some basic points about datums.

  • Latitude and longitude always have a corresponding datum, such as WGS84 or NAD27
  • The latitude and longitude coordinates for your current position may vary across datums
  • Sometimes the difference may be less than a few hundred feet, but it may be much more drastic. WGS84 and NAD83 are nearly identical. The difference between NAD83 and NAD27 varies dependent upon your location within North America. If you're on the U.S. West Coast, the difference between NAD83 and NAD27 could be more than 300 feet. Further East the difference might be less than 100 feet
  • Map projections correspond to a datum
  • When gathering latitude / longitude data for geographic reporting, knowing the datum is important, as you may need to convert your coordinates to the proper datum before using them

Let's look at a basic example using lat/long coordinates for the Golden Gate Bridge.

Golden Gate Bridge Example

The following latitude and longitude coordinates are both valid for the Golden Gate Bridge:

37.819721, -122.478615 in WGS84
37.8197926, -122.4775316 in NAD27

Of the two, only the WGS84 coordinates will display on the Golden Gate Bridge in either Power Map or Power View.

The NAD27 coordinates do not display on the bridge

 The WGS84 coordinates properly display on the bridge

 Here's a comparison of the same coordinates in Power View

The above example provides a scenario where users are likely to notice the geographic point is not correct. That may cause them to not trust the data.

In some cases the difference may not be enough for users to notice. For example, I tried using NAD27 coordinates for several Smithsonian museums in Power Map. The tool displayed the column chart on the correct museum in most cases. The reason being that the museums are fairly large and the shift between WGS84 and NAD27 on the East Coast isn't as big as it is on the West Coast. There were a few Smithsonian locations, though, where the incorrect placement was noticeable.

Thus, when gathering and eventually using latitude and longitude coordinates for geographic reporting, it's important to make sure we know the corresponding datum.

For those interested in more advanced information on datums, you can find more information from Wikipedia here.

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