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coderholic / django-cities

Countries and cities of the world for Django projects
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Region and Subregion model have no Geometry (area|location) - no way to query distance #183

Closed kiddhustle closed 6 years ago

kiddhustle commented 6 years ago

Checklist

from django.contrib.gis.db.models.functions import Distance
start_place = City.objects.filter(country__name='United Kingdom').get(name='London')

result = Region.objects.all().annotate(dist=Distance('location', start_place.location)).exclude(id=start_place.id).filter(name='Scotland').order_by('-dist') or ``result = Region.objects.all().annotate(dist=Distance('location', start_place.location)).exclude(id=start_place.id).filter(name='Scotland').order_by('-dist')

Expected behavior

To return the distance from Scotland region

Actual behavior

Following errors:

FieldError: Cannot resolve keyword 'location' into field. Choices are: alt_names, cities, code, country, country_id, id, name, name_std, postal_codes, slug, subregions
FieldError: Cannot resolve keyword 'area' into field. Choices are: alt_names, cities, code, country, country_id, id, name, name_std, postal_codes, slug, subregions
blag commented 6 years ago

Hi, thank you for reporting this, but this problem is actually much more complex than it might seem at a glance.

If you would like to have a point that represents a region or a subregion: where exactly do you want that point to be? The centroid of the region/subregion? What if that point is someplace far away from any city or sign of civilization? The capital of the region/subregion? Some places have multiple capitals - which one do I pick?

If you're trying to find the distance between two regions/subregions: which points do you want to use? The distance between the centroids? That wouldn't ever really be accurate. The distance between the closest points on each region/subregion? That also wouldn't ever be accurate. The distance between the farthest points on each region/subregion? Also wouldn't ever be accurate.

Really, representing an entire area as a point is the fundamental problem. I probably won't ever implement that for something as large as a region/subregion. There's simply no reasonable and generic way to represent an area as a point.

So lets talk about areas (or "boundaries" in GIS terminology). I would absolutely love to add boundaries to all of the models in the app! However, there isn't really a single good source for that data. Specifically I would like a single source that is:

Furthermore, there is already a PR that attempts to implement this: #159.

Closing, because this is a duplicate of that. Please dig through the previously reported issues and pull requests before opening a new one.

blag commented 6 years ago

This also duplicates issue #125.

kiddhustle commented 6 years ago

Oh OK I see.

I didn't realise that this problem had already been considered in another issue.

I guess in my case I could just take any city that belongs to a region and use that's location. I don't need a super accurate distance, as I only need the distance for sorting as a one off task.

Also because I am running this query on a relatively small dataset (around 100) I can manually correct any results where needs be.

I will have to keep an eye on the project to implement boundaries.

Thanks!

On 17 Nov 2017 06:59, "blag" notifications@github.com wrote:

Hi, thank you for reporting this, but this problem is actually much more complex than it might seem at a glance.

If you would like to have a point that represents a region or a subregion: where exactly do you want that point to be? The centroid of the region/subregion? What if that point is someplace far away from any city or sign of civilization? The capital of the region/subregion? Some places have multiple capitals - which one do I pick?

If you're trying to find the distance between two regions/subregions: which points do you want to use? The distance between the centroids? That wouldn't ever really be accurate. The distance between the closest points on each region/subregion? That also wouldn't ever be accurate. The distance between the farthest points on each region/subregion? Also wouldn't ever be accurate.

Really, representing an entire area as a point is the fundamental problem. I probably won't ever implement that for something as large as a region/subregion. There's simply no reasonable and generic way to represent an area as a point.

So lets talk about areas (or "boundaries" in GIS terminology). I would absolutely love to add boundaries to all of the models in the app! However, there isn't really a single good source for that data. Specifically I would like a single source that is:

  • international - only the US has very good maps (put out by the USGS
  • thank you!), the rest of the world has no good maps
  • accurate - I would like accurate sources
  • legally friendly - the terms and conditions for using some maps preclude them from use in this project - some sources may require payment, even to download or use a sample, or they may require the use of closed source or expensive software to use
  • consistent - one single file format

Furthermore, there is already a PR that attempts to implement this: #159 https://github.com/coderholic/django-cities/pull/159.

Closing, because this is a duplicate of that. Please dig through the previously reported issues and pull requests before opening a new one.

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