[Live-demo] Natural Earth in 4.5b2

[Nathaniel V. Kelso]

As you point out, Natural Earth is mainly focused on cartographic
representation. It should not be used for analytical purposes, but rather to
help portray visual executive summary results of such analysis. If you need
to modify your local copy to resolve such issues, please do so.

_Nathaniel




On Thu, Feb 24, 2011 at 23:21, Hamish <hamish_b at yahoo.com> wrote:

> Nathaniel wrote:
> > I'm not seeing the any geometry errors in ArcGIS (the horror,
> > the horror) on the 10m lakes theme fresh off the website.
>
> Arc is not a topological GIS so it misses (or ignores) many
> errors in the data which cause problems when the data is used
> in other ways which demand topological correctness.
> ([river] network analysis, etc)
> I understand that Natural Earth is mainly focused on cartographic
> needs, but ...
>
>
> > The 1.3 version that was released a couple weeks ago should have
> > resolved most the obvious topology errors on the themes that
> > were ID'd to me before. If you want to run a custom repair
> > geometry / topology script on them as they're imported to the
> > LiveDemo system, sounds fine to me.
>
> Loading a fresh download of 10m-lakes.zip into GRASS GIS (which
> is fully topological) exposes a number of errors in the data.
> Probably most are due to overlapping areas; small sliver areas
> resulting from those after "flattening" are appear below as
> areas without centroids (no attributes).
>
> GRASS65> v.in.ogr dsn=natearth/10m_lakes.shp out=ne_10m_lakes
> [...]
> Building topology for vector map <ne_10m_lakes_tmp>...
> Building areas...
>  100%
> 1708 areas built
> 1472 isles built
> Attaching islands...
>  100%
> Number of nodes: 2627
> Number of primitives: 4691
> Number of points: 0
> Number of lines: 0
> Number of boundaries: 4691
> Number of centroids: 0
> Number of areas: 1708
> Number of isles: 1472
> Number of areas without centroid: 1708
> -----------------------------------------------------
> Find centroids for layer: 10m_lakes
> -----------------------------------------------------
> Write centroids:
>  100%
> WARNING: 82 areas represent more (overlapping) features, because polygons
>         overlap in input layer(s). Such areas are linked to more than 1
>         row in attribute table. The number of features for those areas is
>         stored as category in layer 2
> -----------------------------------------------------
> 1358 input polygons
> Total area: 1.26155E+12 (1708 areas)
> Overlapping area: 2.02361E+11 (82 areas)
> Area without category: 2.39549E+10 (240 areas)
> -----------------------------------------------------
> Building topology for vector map <ne_10m_lakes>...
> Registering primitives...
> 3489 primitives registered
> 138259 vertices registered
> Building areas...
>  100%
> 1708 areas built
> 1472 isles built
> Attaching islands...
>  100%
> Attaching centroids...
>  100%
> Number of nodes: 3253
> Number of primitives: 3489
> Number of points: 0
> Number of lines: 0
> Number of boundaries: 2021
> Number of centroids: 1468
> Number of areas: 1708
> Number of isles: 1472
> Number of centroids outside area: 3
> Number of duplicate centroids: 5
> Number of areas without centroid: 248
>
> #extract problems to another layer:
> GRASS65> v.extract in=ne_10m_lakes out=ne_10m_lakes_problems layer=2
>
> displaying that extract shows problems are all in the northern
> hemisphere, the Great Lakes of N.America being the biggest
> pile of red ink. (Lakes Ontario and Erie look ok though)
>
> a map layer created with "v.clean error=problem_areas_map" can
> help quickly pinpoint where the problems are.
>
> e.g. an easy place to start looking are the finger lakes within
> this bounding box, all of which show errors:
>
> north:      43N
> south:      42:18N
> west:       77:24W
> east:       76:12W
>
> (south of Rochester-Syracuse, NY)
>
>
> best,
> Hamish
>
>
>
>
>
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