I would like to use a hybrid metric to draw isochrones in sDNA. what are the possible ways, and are there any difference in the results?


Yes, there are a few ways:

  1. sDNA Specific Origin Accessibility Maps will compute your metrics (whether hybrid or other types) from the origin(s) you designate to everywhere on the network. Results will be saved as a data field on each individual link. Colouring the output network by this field may be sufficient for your purpose, or if you wanted a polygon output you would have to postprocess by grouping links into different bands of metric, merging and finding bounding polygons.

  2. Polygons can be produced directly for each radius specified using the sDNA Convex Hull tool; though as the name suggests these are convex hulls so anywhere an isochrone is concave in shape it will misrepresent the shape. This may or may not be an issue depending on the desired purpose.

  3. sDNA Network Radii will work as convex hulls, but produce output as a separate multipolyline for the network within each band. This solves the issue for concave isochrones.

Note that in both cases spatial accuracy is limited to the size of links; wherever an isochrone falls on a link it will be represented at the link's center point. If you want greater precision then

  1. with specific origin accessibility maps you will need to split links into multiple pieces

  2. with convex hulls or network radii if you and are using an angular/euclidean/custom metric it's easier, you can get exact precision by enabling continuous space mode, though this isn't currently supported for hybrid metrics (for which you would just have to subdivide links as above).

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