Update README.md (matching algo + mvn) (#91)

README.md

@@ -52,6 +52,9 @@ gps_accuracy=15              # default=15, type=int, unit=meter, the precision o
 
 This will produce gpx results similar named as the input files.
 
+Developer note: After changing the code you should run `mvn clean` before running `map-matching.sh`
+again.
+
 ### UI and matching Service
 
 Start via:
@@ -122,4 +125,20 @@ like when updating the OSM data.
 
 ### About
 
-See [this project](https://github.com/bmwcarit/hmm-lib) from [Stefan](https://github.com/stefanholder) which is used in combination with the GraphHopper routing engine and is used as the algorithmic approach now. Before it was [this faster but more heuristic approach](https://karussell.wordpress.com/2014/07/28/digitalizing-gpx-points-or-how-to-track-vehicles-with-graphhopper/).
+The map matching algorithm mainly follows the approach described in
+
+*Newson, Paul, and John Krumm. "Hidden Markov map matching through noise and sparseness."
+Proceedings of the 17th ACM SIGSPATIAL International Conference on Advances in Geographic
+Information Systems. ACM, 2009.*
+
+This algorithm works as follows. For each input GPS position, a number of
+map matching candidates within a certain radius around the GPS position is computed.
+The [Viterbi algorithm](https://en.wikipedia.org/wiki/Viterbi_algorithm) as provided by the
+[hmm-lib](https://github.com/bmwcarit/hmm-lib) is then used to compute the most likely sequence
+of map matching candidates. Thereby, the distances between GPS positions and map matching
+candidates as well as the routing distances between consecutive map matching candidates are taken
+into account. The GraphHopper routing engine is used to find candidates and to compute routing
+distances.
+
+Before GraphHopper 0.8, [this faster but more heuristic approach](https://karussell.wordpress.com/2014/07/28/digitalizing-gpx-points-or-how-to-track-vehicles-with-graphhopper/)
+was used.