Population count with overlapping isochrones


#1

Dear Nils,
First of all, let me congratulate you and thank you for your work. It has been very helpful for me and certainly for a lot of researchers out there.
I’m currently using the ORS Tools plugin for Qgis to draw isochrones around a set of points of service (hospitals), to do an accessibility/reachability analysis. For this purpose, the data of the population count within each isochrone (which I call ‘service area’) that is stored in the attributes table of the isochrones layer is crucial, and I’m very pleased that you made this upgrade in 2018.
However, there is an issue on which I want to ask for your help: the network of hospitals I’m analyzing is a layer with 31 points scattered over the territory of a country, and many of these points are clustered in urban areas; because of that, there are many cases were isochrones overlap, i.e., an isochrone around point A will intersect an isochrone around point B. Sometimes the overlapping is almost complete, and sometimes just a fraction of the isochrones is overlapped.
The way I interpret the results of the isochrones drawn with QRS Tools is that, taking the sum of the population counts of all isochrones will duplicate (or even more) the counting of the population that resides in the areas of overlapping isochrones. And this is a problem than I need to avoid in order to make the accessibility analysis. Bottom line, the research question I’m trying to answer is: how many people is residing within x minutes of any hospital of the network.
Is there any way to configure ORS Tools to do this, or any workaround or post analysis that could give the result I’m searching?
Thanks in advance for any help.
Cesar Carneiro.


#2

Right, I see your problem. Unfortunately there is no easy way to avoid this scenario. Not on our side anyways.

The best you can do is to download the population source yourself, request the isochrones, neglect the total_pop property and do a raster statistical analysis with isochrone geometry on the population raster you have now on your local machine. To get rid of overlapping areas, you just dissolve all of the isochrone geometries (you might have to a geometric difference of the ranges in case you’re looking at multiple time/distance ranges at once and then dissolve each range separately).

Find the source here:
http://cidportal.jrc.ec.europa.eu/ftp/jrc-opendata/GHSL/GHS_POP_GPW4_GLOBE_R2015A/GHS_POP_GPW42015_GLOBE_R2015A_54009_250/V1-0/

From GHSL:
https://ghsl.jrc.ec.europa.eu/ghs_pop.php

Careful, it’s 1 GB zipped.


#3

Dear Nils,
Thank you so much for your quick reply, and sorry for my late feed-back (been away on vacation).
I think I understand the procedure you’re suggesting, but I’m not sure I will be successful. I will work on this on the next days and I will post my success or failure story here.
César.


#4

Hi Nils.
I followed your suggestion and it worked perfectly.
I didn’t used the raster layer you referred to because I only needed the population in Europe, and so I found this other file: https://cidportal.jrc.ec.europa.eu/ftp/jrc-opendata/GHSL/GHS_POP_EUROSTAT_EUROPE_R2016A/GHS_POP_SOURCE_EUROPE_R2016A_3035_100/V1-0/
I used the ‘dissolve’ algorithm for dissolving the overlapping polygons in the layer of isochrones and ‘zonalstatistics’ for counting population of the raster layer that was under the polygon of the layer that resulted from the ‘dissolve’.
The result is exactly what I wanted.
Thank you so much.
César.


#5

Great, happy to hear that:) Didn’t know they also offered regional extracts, good to know.

Thx for reporting!!
Nils