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

[Wayne Emerson, Jr.]

On 12/30/2019 7:27 PM, Greg Troxel wrote:
> Interestingly I see a small consistent shift between the new layer and
> OSM, in that map.
>
> I am not clear how the property map is dealing with the two datums.
> (None of this datum stuff is easy....)

I first noticed this shift about a year ago when the iD editor software 
switched to using a different TMS server for the L3 parcel map. I later 
saw this email talked about the change of tile servers: 
https://lists.openstreetmap.org/pipermail/talk-us-massachusetts/2018-November/000424.html 


At the time I was not subscribed to this list so I posted about it here:

https://forum.openstreetmap.org/viewtopic.php?id=64699

Not sure if my conclusions there were correct however.

I had assumed that the downloadable files from MassGIS stayed fixed at 
the original datum, while the TMS applied some sort of techtonic plate 
movement based correction. The yearly divergence rate noted in my forum 
post is not big enough to account for the difference we see today, and 
may be partly due to the initial error in the original datum. Probably 
only MassGIS would know the source(s) of the difference.

>
> (I see what you mean about Braintree.)

Looks like mainly north half of Braintree is a mess. These imports were 
done by crschmidt in January 2009.

These are some of the changesets I found on bad Braintree buildings but 
It looks like the changesets cover huge areas:

https://www.openstreetmap.org/changeset/727079

https://www.openstreetmap.org/changeset/743822 
<https://www.openstreetmap.org/changeset/727079>

https://www.openstreetmap.org/changeset/743003 
<https://www.openstreetmap.org/changeset/727079>

The later imports done in 2013 look good.

-Wayne

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