Better completion of fragmentary river networks with the induced terrain approach by using known non-river locations Tsz-Yam Lau and W. Randolph Franklin Rensselaer Polytechnic Institute partially supported by NSF grants CMMI-0835762 and IIS-1117277 SDH 2012

Outline Background Contribution Motivation of river reconnection The induced terrain concept Contribution Easy incorporation of known non-river locations to improve accuracy of river reconnection problem SDH 2012

River reconnection problem 2. Ground surveys are expensive, time-costly and sometimes impossible. 1. The complete river network is needed to address transportation problems 4. Automatic river location identification is incomplete (black). We need to reconnect the segments (red). 3. There are problems with remote multispectral imaging: obstacles, varied reflectance of water SDH 2012

The induced terrain approach Use given heights to eliminate invalid reconnections Model given river locations as local minima w.r.t. non-river locations R Guarantee given river locations must still be river locations SDH 2012

Improvement with additional known non-river location information We should not let water flow there. Ground truth river network Ground truth river network SDH 2012

Improvement with additional known non-river location information A few river flow derivation algorithms have already offered relevant options. (e.g. the –blocking option of r.watershed in GRASS GIS) But it could be unreasonable to expect every implementation to have such an option. There is no way to add that feature to a closed-source implementation. It is difficult to realize similar things even if we have the source code. SDH 2012

Original induced terrain Height point samples and partial river Hydrologically corrected terrain SDH 2012

Adaptation to known non-river locations Blue: known river locations Heights of known non-river locations raised Terrain from last step White: known cloud locations SDH 2012

How many should we raise? for known non-river locations Objective No water at given river locations should flow to known non-river locations. It should be effective with many different river derivation algorithms. Different kinds of river derivation algorithms Flow assignment based on local relative heights Flooding before local flow assignment Least-cost search Otherwise SDH 2012

How many should we raise? for known non-river locations Otherwise Objective No water should flow to known non-river locations. It should be effective with many different river derivation algorithms. Different kinds of river derivation algorithms 25 30 22 10 5 15 25 30 22 25 30 22 25 30 22 11 14 13 10 5 15 10 10 15 10 20 15 25 30 22 11 14 13 11 14 13 11 14 13 10 5 15 Flow assignment based on local relative heights (e.g. single-flow direction) 11 14 13 Flooding before flow assignment Least cost search SDH 2012

How many should we raise? Raising by the maximum height in the terrain Flow assignment based on local relative heights Locations higher than those locations are now places with no water. So they can never receive any water. Flooding before local flow assignment: The level raised can never be higher than those non-river locations since they are supposed to leave from other places Least-cost search: Known non-river locations are guaranteed to find inflow cells after all other uncertain locations. In other words, they will never find inflow cells which has water inflow. SDH 2012

Improvement with additional known non-river location information No more reconnections at known non-river locations SDH 2012

Improvement in reconnection accuracy Around 5 percentage point improvement SDH 2012

Conclusion Contribution of our work Avoid reconnection of segments using regions known to have no river flow by setting the heights of known river locations to an unreachable level Need not change anything in the river derivation algorithm Improve accuracy by 5%. SDH 2012

Further work Incorporate other terrain information (e.g. segment geometry) to improve accuracy further Port the induced terrain framework to completion of 3D dendrite networks SDH 2012

Questions? SDH 2012