[Talk-us-massachusetts] Braintree buildings+(Datum shift)

[Andy Anderson]

Sorry, but I strongly disagree. NAD83 and WGS84 were never considered equivalent by people who understood what they meant. I have been teaching this point for almost two decades now. See the attached image. It’s a warning from ArcGIS about this issue that’s been present in the software for at least that long.

It is quite often that the difference doesn’t matter for a particular application, but that is not the same thing as being equivalent. This is basically what the PSU web page says: “we did not have to be concerned with the shift between NAD83 (1986) and WGS84 as introduced in 1987, because the discrepancy easily fell within our overall error budget.” But PSU would also tell you that even then one should not be using NAD83 outside of North America, because the error would be much larger. Or to use it for survey-level measurements.

So it is a best practice in the GIS community to always specify the datum being used for a particular set of geographic data. And if they are not specified, I always tell my students to try to verify the datum with the provider. Then they can judge what transformations might be necessary.

— Andy

> On Dec 31, 2019, at 9:18 AM, Greg Troxel <gdt at lexort.com> wrote:
> 
> Andy Anderson <aanderson at amherst.edu> writes:
> 
>> As you note, NAD83 and WGS84 are two different models of the
>> Earth. I’ll elaborate from what I know.
> 
> Good points, but it's very important to realize that saying "NAD83" or
> "WGS84" without specifying which realization is needlessly imprecise.
> Basically, the first realization of each were considered equivalent, and
> each has become more precise differently and the recent realizations are
> not considered equivalent.
> 
> Sorry if this is a dup link, but:
> 
>  https://www.e-education.psu.edu/geog862/node/1804

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