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

[Andy Anderson]

Agreed, OSM is the main concern as it is a hodgepodge of data from different datums. But also there are many other sources of error, too. There’s probably still a lot of Tiger data (intended for topological rather than geometric consistency), and image extraction is rife with errors. So we shouldn’t set our expectations too high, other than for improvement over time.

Here’s MassGIS’ definition of its NAD83 spacial reference:

	https://www.mass.gov/info-details/overview-of-massgis-data#spatial-reference-of-massgis-data-

The spheroid hasn’t changed (GRS80), and no reference to the coordinate system shifts that have occurred. They probably are using NAD83 (2011) for more recent data,  but that may not cover everything there. Since the difference is centimeter scale, as you say they are probably waiting for 2022 for any major update.

— Andy

> On Dec 31, 2019, at 10:36 AM, Greg Troxel <gdt at lexort.com> wrote:
> 
> On 2019-12-31 10:25, Andy Anderson wrote:
>> 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.
> 
> I meant that NAD83(1986) and WGS84(original) were considered close enough for most purposes in North America.  I understand that for geodetic surveying, you have to pay more attention.   Also there is the point that there are no precise means to access WGS84, for pretty much anyone.  You can get NAD83(2011) and ITRFsomething now from CORS, but I am unaware of a significant number of reference stations that publish coordinates in WGS84.
> 
>> 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.
> 
> Sure (especially about outside NA and surveying).  But it is telling that there are no published transforms, when NGS publishes transforms for many things.   To me that sounds like the transform differences are on the order of the error estimates of the transforms.
> 
>> 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.
> 
> Absolutely.   But these days, that means calling out anyone that says "WGS84" without specifying a realization.
> 
> 
> (I think we are basically in agreement and debating the very edges.)
> 
> To me the big deal is clarifying what OSM coordinates are supposed to mean, and being careful about transforming MassGIS data.   I am assuming until further clarification that MassGIS data is intended to be in NAD83(2011), and that OSM data should be in WGS84(G1762).   And I am assuming that even though epoch cannot be neglected that OSM is not ready to cope and that trying to is pushing a wet noodle uphill.
> 
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