Improving river network completion under absence of height samples using geometry-based induced terrain approach Tsz-Yam Lau and W. Randolph Franklin Rensselaer.

Improving river network completion under absence of height samples using geometry-based induced terrain approach Tsz-Yam Lau and W. Randolph Franklin Rensselaer Polytechnic Institute partially supported by NSF grants CMMI and IIS In this presentation, I am going to talk about how river segment geometry can be used to further improve the induced terrain approach of completing fragmentary river networks. Don’t be scared by the long title. The thing that we are doing is that, given the broken river segments on the left, how we can link them up to form a complete river network, as shown on the right, in an automated manner. We have broken segments because automated analysis of aerial photos is hindered by view obstacles such as clouds or tree canopies, and non-unique reflectance spectrum of water at certain locations. However, a complete river network is necessary to solve cross-terrain transportation issues such as ship routing, pollutant monitoring and flood plain control. Sept 18, 2012 Autocarto Lau & Franklin

Autocarto 2012 Lau & Franklin
Broader Impact Better real-time monitoring of rapidly-changing hydrography with a huge set of aerial photographs captured from time to time The broad theme of our work is on the automated linking of the broken segments. Automation of the linking process is necessary if we wish to monitor the rapidly changing hydrography which is more and more likely to occur due to the recent climate change. This helps in, for example, real-time flood plain control. Since a huge set of aerial photographs is captured from time to time, it doesn’t make sense to hire people to do the linking. Sept 18, 2012 Autocarto Lau & Franklin

Contribution Enhance the induced terrain approach with river segment geometry to further improve automated river reconnection accuracy In our paper, we discuss how the induced terrain approach could be enhanced with river segment geometry to further improve automated river reconnection accuracy. In this presentation, I will first briefly go through the induced terrain approach which was our previous solution to the automated segment linking problem. Then I will discuss river segment geometry in details, before presenting how much improvement we can achieve with this new piece of knowledge. Sept 18, 2012 Autocarto Lau & Franklin

The induced terrain approach
(Lau and Franklin, 2011) So first the induced terrain approach. This video shows the whole induced terrain process. First we reconstruct a terrain surface from the partial height point cloud. Working out this way, the partial height information is readily used in the subsequent river reconnection step to eliminate reconnections that clearly violate the constraints by the partial heights. For example, a hill sitting between two river segments could act as an obstacle to block water from one segment to flow to another segment. Also note that we do a hydrological correction afterwards, which basically model the given river locations as local minima, so as to increase their capability to trap water there, a terrain feature that we expect at a river location. In the second step, we fill in given river locations with an amount of water sufficient for them to become a river location. Then we use a river derivation scheme to reconnect the segments. In general, a river derivation scheme forms rivers in such a way that it takes lowest cost for water to flow out from the terrain. Meanwhile, the particular scheme used guarantees that the reconnections follow our expectation on the river network topology. For example, by assigning water flow to a single neighboring cell, we make sure that no loop is allowed in the network. In contrast, if we allow water from a cell to be distributed across multiple neighboring cell, we may find loops in the resultant network. After all, reconnections that violates given information are eliminated, thus increasing the chance for true reconnections to be recovered. As a result, this approach features higher accuracy over conventional approaches, which treat the issue as a typical line-joining problem ignoring those two important constraints. Sept 18, 2012 Autocarto Lau & Franklin

Missing partial heights: obstacles
In this presentation, we focus on the situation in which we don’t have sufficient reliable height samples. This could occur during summer when the tree canopies can grow so well that the height-detection laser can hardly reach the ground. Sept 18, 2012 Autocarto Lau & Franklin

Missing partial heights: flat surface
(LeFavor and Alsdorf , 2005) This could also happen at flat surfaces such as those in Amazonia. With a slope of just 2.7cm per km, a small noise in a few measured height values could change the topology of the reconstructed river network drastically. Amazon River basin-wide water-surface SRTM C-band heights (blue dots). A 3rd order polynomial fit of the data (green line) and with its slope (red line). Sept 18, 2012 Autocarto Lau & Franklin

Baseline terrain model
V shapes centered at given river locations In order to enjoy the hydrological consistency enforcement, we still recommend the induced terrain approach for the reconnection process. However, we need to figure out some other way to model the terrain surface. Knowing the given river locations are where water is trapped, it is natural to assume that they have the lowest heights in the terrain. Other locations are trying to move their water to their respective nearest river location. This is equivalent to considering each river location as the center of a V shape. The final terrain is the minima of all the V shapes. Sept 18, 2012 Autocarto Lau & Franklin

Favoring shortest-path reconnections
A pair of river locations distant further apart has a higher cost to be connected. Note the implication of such a terrain model: for a pair of river locations closer with each other, the water from one location needs to overcome a smaller total increasng height in order to reach the other location. This favors connection between them rather than another river location further apart. Known river locations x x x Sept 18, 2012 Autocarto Lau & Franklin

A pair of river locations distant further apart has a higher cost to be connected. Difficult Easy x x x Sept 18, 2012 Autocarto Lau & Franklin

Pros: Match human heuristics of linking segments with shortest length Shortest length, lowest cost This matches a common human heuristics that links segments with shortest length and hence lowest cost. outlet outlet Sept 18, 2012 Autocarto Lau & Franklin

Pros: Match human heuristics of linking segments with shortest length Shortest length, lowest cost outlet outlet Sept 18, 2012 Autocarto Lau & Franklin

Cons: Ignore “extend from tips” heuristic outlet However, this practice ignores a few other common heuristics. First, it is the “extend from tips” heuristic. outlet Sept 18, 2012 Autocarto Lau & Franklin

Cons: Ignore “extend from tips” heuristic outlet Reconnection with baseline model With the baseline terrain model, we probably link the middle segment this way so as to route all water to outlets. outlet Sept 18, 2012 Autocarto Lau & Franklin

Cons: Ignore “extend from tips” heuristic outlet Expected extension directions However, when a human being faces this problem, it will look for extensions starting from the segment tips. outlet Sept 18, 2012 Autocarto Lau & Franklin

Cons: Ignore “extend from tips” heuristic outlet Expected reconnection This leads to an alternative yet much more natural reconnection. outlet Sept 18, 2012 Autocarto Lau & Franklin

Cons: Ignore “Join segments which faces each other” heuristic outlet Another heuristic being ignored with the baseline model is so-called “join segments which faces each other”. Given this set of segments outlet Sept 18, 2012 Autocarto Lau & Franklin

Cons: Ignore “Join segments which faces each other” heuristic outlet Reconnection with baseline model The baseline model will make reconnection to the right outlet, which is rather odd. outlet Sept 18, 2012 Autocarto Lau & Franklin

Cons: Ignore “Join segments which faces each other” heuristic outlet Expected reconnection In contrast, what we expect is a link to the left outlet. outlet Sept 18, 2012 Autocarto Lau & Franklin

Cons: Ignore “replicate straightness behavior in the segment extension” heuristic outlet outlet One more heuristic that is being ignored is the “replicate straightness behavior in the segment extension”. Suppose we are given the following set of segments. Sept 18, 2012 Autocarto Lau & Franklin

Cons: Ignore “replicate straightness behavior in the segment extension” heuristic outlet outlet With the shortest-path reconnection scheme, we have the following reconnection. Reconnection with baseline model Sept 18, 2012 Autocarto Lau & Franklin

Cons: Ignore “replicate straightness behavior in the segment extension” heuristic outlet outlet However, if we consider the curving behavior of the segment, it appears more natural to connect it the other way. Expected reconnection Sept 18, 2012 Autocarto Lau & Franklin

Improvement Reduce the rate of height increase at locations radiated from segment tips To embrace those heuristics, we purpose reducing the rate of height increase at locations radiated from segment tips. Given a river segment, we first estimate the respective forward directions as shown in red lines. We then form privileged zones which span theta degree from each of these forward directions. Those regions will have height growth rate sigma dash which is smaller than the regular growth rate sigma at other regions. Sept 18, 2012 Autocarto Lau & Franklin

Parameter setting:  Determine the bending that we accept for privileged connections of mutually facing segments Give good results with /4 or /8 on average. There are two parameters in the modified terrain models, theta and sigma dash. Theta determines the bending that we accept for privileged connections of the mutually facing segments. We find that on average, good results are found with /4 or /8. Sept 18, 2012 Autocarto Lau & Franklin

Parameter setting: ’ Control to what extent we favor height growing according to segment’s straightness over proximity to river locations Give good results with 0.5  on average. Sigma dash controls to what extend we favor height growing according to segment’s straightness over proximity to river locations. The smaller it is with respect to the typical height growing rate sigma, the more favorable it is. On average case, we find 0.5 sigma being a good value. Sept 18, 2012 Autocarto Lau & Franklin

Results This table shows the reconnection accuracy with different terrain models, baseline, tip-biased, and NN-SB which makes use of rich height samples. With the tip-biased model, the reconnection accuracy is improved by 5.2 percentage points. When compared with the case with rich height samples, which gives 13.5 percentage point improvement, our scheme successfully 40% of what we can correct with rich height samples. 40% of what we can correct with rich height samples (density = 10%) Sept 18, 2012 Autocarto Lau & Franklin

Conclusion Adjust the probability of receiving reconnection of different parts of the river segments Shortest path is no longer the single criterion to determine how segments are reconnected Recover 40% of what can be achieved with rich height samples (density = 10%) So here comes the conclusion. The major contribution presented in this paper is to avoid reconnection of segments using regions known to have no river flow, by raising their heights to an unreachable level. We need not change anything in the river derivation algorithm, which could be difficult or simply impossible to do. After all, we manage to improve reconnection accuracy by 5 percentage point. Sept 18, 2012 Autocarto Lau & Franklin

Future work Port the induced terrain framework to completion of 3D dendrite networks As for future work, we will look into whether the same framework can be used to complete some other networks such as dendrite networks. Sept 18, 2012 Autocarto Lau & Franklin

Questions? So that’s all for my presentation. Any questions? 40% of what we can correct with rich height samples (density = 10%) Sept 18, 2012 Autocarto Lau & Franklin

Improving river network completion under absence of height samples using geometry-based induced terrain approach Tsz-Yam Lau and W. Randolph Franklin Rensselaer.

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Improving river network completion under absence of height samples using geometry-based induced terrain approach Tsz-Yam Lau and W. Randolph Franklin Rensselaer.

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Presentation on theme: "Improving river network completion under absence of height samples using geometry-based induced terrain approach Tsz-Yam Lau and W. Randolph Franklin Rensselaer."— Presentation transcript:

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