Geo.Hackmin Week, 01.-06. March 2021

  • BA & MA in Physics
  • PhD in Computational Neuroscience
  • now at Department of Physiology, Uni Bern; modeling of plasticity & learning in brains
  • so far: consumer of geo data
  • why this hackathon?
    • learn something
    • build something
    • open data is awesome!

twitter: @_jakobj


About me

Locate crosswalks in Bern

The goal

with the help of aerial photographs


The method

Supervised machine learning

-> need (lots of) examples!

(46.94926, 7.45258,  ...)

(46.94984, 7.45432, ...)

(46.94836, 7.45973, ...)



Gathering data

(Semi)manual annotation of full-scale images

-> extraction of ~1000 positive examples (50x50px)

(negative samples were generated automatically & reviewed)

Training the machine

Small convolutional neural network

(2x [Conv2d + ReLU + MaxPool2d] + 1x FC)

Output: probability that input image patch contains  crosswalk

~20k parameters, ~40min training time, ~150 loc

Exporting the data (1/2)

positions of crosswalks within the photograph in pixels


coordinates of each photograph


(+ merging regions of interest)

Exporting the data (2/2)

~2500 ROIs imported into

Lessons learned

  • time management
    • pick a project that's realistic, a week is short!
    • making some choice may be better than making the best
  • applied machine learning
    • consider the difficulty of the classification task; you may not always need a model trained on ImageNet
    • poor performance is likely an issue of the data, rather than the model; check (and retrain) missclassifications!
    • data augmentation FTW!
  • geo data: coordinate transformations are a pain

Closing remarks

  • simple supervised-ML methods are suitable for detecting regions of interest in aerial photographs
  • (likely) every single design choice in this project can be improved
  • give it a try! code & some doc (MIT license):

Thanks to everyone involved for making this possible!

It was awesome!

Computer symbol: PanierAvide (Creative Commons)

Student photo: CollegeDegrees360 (Creative Commons)



By jakobj


Detect pedestrian crossings (or other persistent structures) in the SWISSIMAGE 10 cm dataset.

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