[OSM-talk] All you've ever wanted to know about the french cadastre

[Richard Weait]

On Wed, Sep 26, 2012 at 12:59 PM, Olivier Croquette <ml at ocroquette.de> wrote:

> If no, it doesn't make any sense to me that a vector based process for the cadaster is an import, and a raster based is not. Everything is the same : kind of data, license, provider…
>
> There seems to be a contradiction there.

Yes, this is something of a contradiction or edge case.  May I offer,
from my perspective, an important difference between tracing over
raster, and copy / pasting vectors?  You say:

"Some cities (10% as an order of magnitude) have only a raster
cadaster, in which case the mapper has to draw all the nodes and
points manually."

I think that "drawing all of the nodes and points manually" is an
important difference, from a quality point of view.  Each node or way
that you draw by hand, is carefully considered and placed, one at a
time.  It isn't perfect; nothing is.  I suggest that this leads to a
kind of automatic quality control, as the nodes and ways are placed.

The goal of the vector import procedure is similar, use data from this
area, reconcile it carefully, include it in OpenStreetMap.  The
intention is very good.  But in execution, it is easier to miss a node
or way (or more than one) that needs to be refined before upload.
Again, it isn't perfect; nothing is.  When you are considering
hundreds or thousands of nodes and ways at once, it becomes time
consuming to check them all.

I hope that you'll find the above to be easy to agree with.

My conclusion, is this.  The quality of the hand drawn nodes and ways
will be better because when we draw the nodes and ways by hand they
get more individual attention and care than when we start with a group
of nodes and ways from another file.

So that's why I think that it is different to trace by hand, vs.
vector import.

But what about the rules and edge cases?

I consider what you describe as the raster process to be an import
when the quantity of data is large.  The raster process relies on an
external data source for a large quantity of information.  If that
source is used without considering additional data sources, I think
that the classification as import is very clear.

On the other hand, I think that if the quantity of data is small, if
multiple sources are considered with appropriate weight, and if local
knowledge or an in-person survey is included as well?  Then the
description might be closer to "really good mapping."

You ask what is the difference between the raster process and tracing
from aerial imagery alone.  Good question.  We haven't to this point
considered aerial tracing to be an import, but perhaps we should.
Perhaps the reason that tracing aerial imagery is not-an-import is
because it is transformative in a more obvious way?  Tracing aerial
imagery transforms from a (rectified and positioned) picture of the
real world, to a vectorized and tagged abstraction.  Tracing the
raster procedure, if I understand it correctly, transforms from a
raster version of one vectorized and tagged abstraction to second
vectorized and tagged abstraction.

I'd like to repeat here, something that I've said elsewhere.  I think
that the stated goal of the cadastre process, as I understand it, is
admirable.  The idea that data from an external source can not be
contributed to OSM until it has been merged with full consideration of
existing data and (all) other available sources, seems exactly the
right approach.  I hope that this requirement is adopted more widely.