Cartogram of countries of the world display error

Question

Cartogram of countries of the world display error

neuronwave opened this issue 11 years ago · 16 comments

When creating a cartogram for the countries of the world, countries sitting on or passing through the right side of the map e.g. 180deg distort when the cartogram is created.

It appears that the geometry array that is created has negative (or zero) values for components of the features path and this is causing the display issue.

I have created a cut down example at http://flinklabs.com/labs/worldcarto/ which highlights the issue.

It is possible to partially correct this by overwriting the values for Russia and Fiji but this is not perfect nor ideal.

Answer 1 · 2013-10-01T07:03:46.000Z

Ensure you have the latest build of topojson which fixes a bug related to
this, and ensure you're using the default countries file, not the
scale-rank version to provided good input into topojson.

On Mon, Sep 30, 2013 at 11:58 PM, benhosken notifications@github.comwrote:

When creating a cartogram for the countries of the world, countries
sitting on or passing through the right side of the map e.g. 180deg distort
when the cartogram is created.

It appears that the geometry array that is created has negative (or zero)
values for components of the features path and this is causing the display
issue.

I have created a cut down example at http://flinklabs.com/labs/worldcarto/which highlights the issue.

It is possible to partially correct this by overwriting the values for
Russia and Fiji but this is not perfect nor ideal.

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Answer 2 · 2013-10-01T07:33:57.000Z

Thanks Nathaniel for the good suggestions. I have downloaded the latest version from github v 1.4.2 and am using the very latest world-110m.json file from the topojson github repro as well. After changing to these, there was no difference in the display. The example showing the issue has been updated with the latest files.

Answer 3 · 2013-10-01T14:38:31.000Z

Link to the countries file?

Sent from my handsful device.

On Oct 1, 2013, at 0:33, benhosken notifications@github.com wrote:

Thanks Nathaniel for the good suggestions. I have downloaded the latest
version from github v 1.4.2 and am using the very latest world-110m.json
file from the topojson github repro as well. After changing to these, there
was no difference in the display. The example showing the issue has been
updated with the latest files.

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Answer 4 · 2013-10-01T16:37:05.000Z

I don't think that this is a TopoJSON issue, but a limitation of the original algorithm. There's nothing in the algorithm to "pin" points to their original location, because its goal is to shrink or grow each polygon (by moving its points) to have an area proportional to its numeric value. I can imagine some hacks that would achieve this purpose, but they would have strange side effects. For instance, if you pin one arc (edge) of a polygon, the rest of its points will be distorted more aggressively to modify the polygon's area accordingly.

The other issue that might be at play here is that MultiPolygon geometries with very large bounding boxes (for instance, if Russia is split by the date line) may be dramatically distorted. I think the only way to do this right with a world map is to ensure that countries are combined, e.g. by using another projection that doesn't break up Russia. @nvkelso, @mbostock or @jasondavies might have some ideas here... Perhaps a Dymaxion™ projection?

Answer 5 · 2013-10-01T16:43:46.000Z

Stitching of MultiPolygons (e.g. Russia) hasn’t actually made it to a release yet: see topojson/topojson#119 for the work-in-progress.

Answer 6 · 2013-10-01T16:58:04.000Z

Sweet, thanks Jason. I'll keep this issue open and we'll see if stitching together Russia addresses it.

Answer 7 · 2013-10-01T22:28:31.000Z

Thanks everyone. I'll keep an eye out for when the stitching makes it in. In the meantime, as you suggest Shawn, there are some hackery pokery things we can/have done to restrict the wraparound occurring once the geometries have been returned from cartogram.js.

Answer 8 · 2014-01-26T19:55:11.000Z

Slightly off-topic, but any ideas why the countries and continents look so strange? E.g. the USA is extremely small. What values should I apply instead of the hardcoded 100 in the original example to get a "normal" map? If I weight countries by the territory (ha), then Africa would be distorted. Any idea?

Answer 9 · 2014-01-27T17:50:56.000Z

I was actually thinking about this recently. The multiplier just amplifies the area (in projected pixels) of each feature linearly. What we (the d3-cartogram internals) should probably use instead is the average feature area. I'll take a look and get back to you.

Answer 10 · 2014-01-27T21:31:58.000Z

In the project we are using Cartogram.js for we calculate the z-score across all the countries and then multiply that by a factor k (I think 100) and then push that into the library to generate the sizings. We tried using a spread i.e. 0 for the smallest 100 for the largest but this skewed many the visual display too much.

When we want to get a base map from cartogram.js we set the iterations to 1 and then call it. Then reset the iterations to 8, 12 or 16 depending upon the iterations required.

As an aside, at a world map level, we used a simplified map as the level of detail of borders made computation run for too long in our use case.

On 28 Jan 2014, at 4:50 am, Shawn Allen notifications@github.com wrote:

I was actually thinking about this recently. The multiplier just amplifies the area (in projected pixels) of each feature linearly. What we (the d3-cartogram internals) should probably use instead is the average feature area. I'll take a look and get back to you.

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Answer 11 · 2014-01-28T23:55:57.000Z

Thank you @benhosken, setting the iterations to one is working perfectly.

Answer 12 · 2015-04-23T22:21:39.000Z

Hello Everyone,

I am new to d3.js. I am trying to understand the cartogram for world map. Like mentioned here, my cartogram also has lines (Russia). Can anyone tell me what is the solution for this : http://stackoverflow.com/questions/29787205/how-to-remove-lines-in-between-cartogram-maps-in-d3-js

Answer 13 · 2015-04-23T23:18:14.000Z

@benhosken Did u find any solution for this issue ?

Answer 14 · 2015-04-23T23:32:09.000Z

We did find a solution to this which involved manually editing the json map in QGis. The issue was caused with arcs that went from 180 or maybe 360 to 0 basically when they wrapped around the edge.

Because we were creating a cartogram, the accuracy of the shape was not a problem when we had a tiny section of an island cut off.

I've answered the StackOverflow question with an example JSON file we used.

We also simplified the map as we needed to create real time cartograms and then convert them into a hexagonal representation in the browser. Pushing the boundaries quite a bit but it did work.

Let me know if you have problems with it.

Ben Hosken
Founder
Flink Labs
www.flinklabs.com
@flinklabs
03 9018 5017

Flink is clever

On 24/04/2015, at 8:21 AM, Shalini Ravishankar notifications@github.com wrote:

Hello Everyone,

I am new to d3.js. I am trying to understand the cartogram for world map. Like mentioned here, my cartogram also has lines (Russia). Can anyone tell me what is the solution for this : http://stackoverflow.com/questions/29787205/how-to-remove-lines-in-between-cartogram-maps-in-d3-js

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Answer 15 · 2015-04-24T02:02:33.000Z

Hey @shaliniravi,

I've been building a cartogram of the world, and solved it a different way from @benhosken. I used https://github.com/mbostock/world-atlas, and modified the Makefile to add --stitch-poles false. That section now looks like:

topo/world-%.json: shp/ne_%_admin_0_countries.shp
    mkdir -p $(dir $@)
    $(TOPOJSON) \
        --quantization 1e5 \
        --stitch-poles false \
        --id-property=+iso_n3 \
        -- countries=shp/ne_$*_admin_0_countries.shp \
        | $(TOPOMERGE) \
            -o $@ \
            --io=countries \
            --oo=land \
            --no-key

Then when I ran make topo/world-110m.json I got this file, which has the Chukchi Peninsula (and a tiny bit of Fiji) as separate polygons.

Hope this helps!

Answer 16 · 2015-04-24T02:39:34.000Z

I understood. It works fine now :) Thank You Guys 😊

Thanks & Regards,
Shalini Ravishankar

From: Alex Louden
Sent: ‎Thursday‎, ‎April‎ ‎23‎, ‎2015 ‎10‎:‎02‎ ‎PM
To: shawnbot/d3-cartogram
Cc: Shalini Ravishankar

Hey @shaliniravi,

I've been building a cartogram of the world, and solved it a different way from @benhosken. I used https://github.com/mbostock/world-atlas, and modified the Makefile to add --stitch-poles false. That section now looks like:
topo/world-%.json: shp/ne_%admin_0_countries.shp
mkdir -p $(dir $@)
$(TOPOJSON)
--quantization 1e5
--stitch-poles false
--id-property=+iso_n3
-- countries=shp/ne$*_admin_0_countries.shp
| $(TOPOMERGE)
-o $@
--io=countries
--oo=land
--no-key

Then when I ran make topo/world-110m.json I got this file, which has the Chukchi Peninsula (and a tiny bit of Fiji) as separate polygons.

Hope this helps!

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