Re: [API-users] Clipping occurrence data to a country

[Adriaan van Os]

Matthew Blissett wrote:
I've made an issue and a live demo: 
https://github.com/gbif/maps/issues/29 (using vector tile points, as the 
polygon maps are generate from the point sources, and the raster tiles 
from the vector ones).


The Dutch data are peculiar in that a system is used in the Netherlands that counts (not just 
moths) in each 5x5 km square. That has some interesting effects on GBIF maps:


1. The GBIF polygon maps from such data show a Moiré pattern, caused by two rasters interfering 
(GBIF polygons and the Dutch 5x5 squares) unless the polygon size is carefully choosen.


2. The point maps don't show a "natural distribution" in the Netherlands as can be seen in this 
example 


3. Apparently, the Dutch data have an offset and must be shifted to the North-East to align. I 
don't know if this is a GBIF or Dutch data problem. Nor do I know if it is the center of the 5x5km 
squares that is entered as GBIF map position.


Regards,

Adriaan van Os

___
API-users mailing list
API-users@lists.gbif.org
https://lists.gbif.org/mailman/listinfo/api-users

Re: [API-users] Clipping occurrence data to a country (errata)

[Adriaan van Os]

$nx=65..65


I meant to write:
$nx=65..66

Regards,

Adriaan van Os

___
API-users mailing list
API-users@lists.gbif.org
https://lists.gbif.org/mailman/listinfo/api-users

Re: [API-users] Clipping occurrence data to a country

[Adriaan van Os]

Matthew Blissett wrote:

Hi Adriaan,

Could you give an example?


Well, I just started with the A, doing my iteration with "=NL" added to the http request 
and "-nl" addded to the filename. As you can see below, they all compare negative.


























I stopped there, but could list a few hundred more 

I iterate prcedurally

$tURL:="https://api.gbif.org/v2/map/occurrence/density/"+String($nz)+"/"+String($nx)+"/"+String($ny)+"@4x.png?"+$sProjection+$sStyle+"="+$nTaxonKey+$sCountry
$nHttpResponse:=HTTP Get($tURL;$uTile;$atHeaderNames;$atHeaderValues)

where $nz=7
$nx=65..65
$ny=41..43
$sStyle:="=classic.poly=hex=30"
$sCountry:="=NL" (or an empty string)
$sSuffix:="-nl" (or an empty string)
$sProjection:="srs=EPSG:3857"
$nTaxonKey comes from a database table, imported from the GBIF backbone

Regards,

Adriaan van Os
___
API-users mailing list
API-users@lists.gbif.org
https://lists.gbif.org/mailman/listinfo/api-users

Re: [API-users] Clipping occurrence data to a country

[Adriaan van Os]

Matthew Blissett wrote:

For the other approach, I don't have EPSG:3575-format vector tiles with 
country borders, but in any case it's difficult to work with vector 
tiles for any further processing — if you wanted a polygon, you'd need 
to reconstruct it from the lines which are cut off at the edges of the 
tiles. I recommend you use a different source of map data, e.g. from 
http://www.naturalearthdata.com/ . Many GIS libraries (including 
Javascript ones for display) should be able to work with that data, for 
example in GeoJSON format: 


With some programming, the poly-edge cutting problem can be solved. Besides, on large-scale maps 
with small polys, I find cutting the poly edges really improves the looks of the map. Also, one 
could use the country borders to highlight (e.g.) a specific country on the background-map before 
overlaying the occurrence polys. So, I do suggest adding "=" to the GBIF background-map 
tile API.


https://github.com/nvkelso/natural-earth-vector/tree/master/geojson (if 
10m resolution is sufficient).


Thanks !

The Natural Earth Admin0 data sets 
 are what I need. Best of all, it 
can be loaded into QGiS and converts perfectly well to EGCS:3575. There is not much on the internet 
(that I could find) for converting to EGCS:3575. I have only located QGiS, OpenLayers (apparently) 
and Leaflet (Proj4Leaflet c.s.). And GBIF, of course.


The advantage of EGCS:3575 is that it is an equal area projection 
, unlike EPSG:3857 Web 
Mercator. I don't like Web Mercator distortions. I guess EPSG:3857 became polular when Google 
started to use it, thinking that people will be confused when the line pointing North isn't 
vertical. God should have made the earth a cube for that purpose ? Well, that causes some problems 
at the edges when travelling 


I will investigate the =NL issue further and reply seperately.

Regards,

Adriaan van Os

___
API-users mailing list
API-users@lists.gbif.org
https://lists.gbif.org/mailman/listinfo/api-users

Re: [API-users] Clipping occurrence data to a country

[Matthew Blissett]

Hi Adriaan,

Could you give an example?

Just visually, the map tile looks fine when flipping between 
https://api.gbif.org/v2/map/occurrence/density/6/65/1...@2x.png?srs=EPSG:4326=square=32=6=green.poly
 
and 
https://api.gbif.org/v2/map/occurrence/density/6/65/1...@2x.png?srs=EPSG:4326=square=32=6=NL=green.poly

Zooming in one step to the top right (which is only Dutch territory) and 
comparing the vector tiles with and without the country mask:

https://api.gbif.org/v2/map/occurrence/density/7/131/26.mvt?srs=EPSG:4326=square=32=6

https://api.gbif.org/v2/map/occurrence/density/7/131/26.mvt?srs=EPSG:4326=square=32=6=NL

These are byte-for-byte identical.

I don't see a problem in EPSG:3575 either, e.g. with 
https://api.gbif.org/v2/map/occurrence/density/5/15/2...@2x.png?srs=EPSG:3575=square=32=6=NL=green.poly

Note that close to borders, GBIF's processing assumes the source data is 
correct as we know our map isn't perfect. An observation in Westerwoldse 
Aa on the border of Netherlands and Germany could be accepted with 
either country=NL or country=DE, and would show accordingly if the map 
is clipped to NL or DE.


For the other approach, I don't have EPSG:3575-format vector tiles with 
country borders, but in any case it's difficult to work with vector 
tiles for any further processing — if you wanted a polygon, you'd need 
to reconstruct it from the lines which are cut off at the edges of the 
tiles. I recommend you use a different source of map data, e.g. from 
http://www.naturalearthdata.com/ . Many GIS libraries (including 
Javascript ones for display) should be able to work with that data, for 
example in GeoJSON format: 
https://github.com/nvkelso/natural-earth-vector/tree/master/geojson (if 
10m resolution is sufficient).

Cheers,

Matt Blissett

On 08/01/2019 22:19, Adriaan van Os wrote:
> I noticed that e.g. "=NL" limits occurrence data even within 
> the specified country. That is, for that country, I get less 
> occurrence data than without the "=NL"
>
> Is this caused by wrong data, by data with the wrong (or no) country 
> identifier attached ?
>
> Anyway, I now seek another solution, which is to clip the occurrence 
> data to the specified country on the map. However, how do I get map 
> data tiles with country borders ? I use EPSG:3575.
>
> Regards,
>
> Adriaan van Os
> ___
> API-users mailing list
> API-users@lists.gbif.org
> https://lists.gbif.org/mailman/listinfo/api-users
___
API-users mailing list
API-users@lists.gbif.org
https://lists.gbif.org/mailman/listinfo/api-users