Re: [GRASS-user] r.stream.extract Montgomery
stephen sefick wrote: I would like to use the Montgomery method to extract streams (exp~2 from the paper). What should the threshold value be? And in general what should the threshold value be? I am working in the southeastern coastal plain (USA), which is characterized by low gradient (if this helps). I am using 1m resolution LIDAR data. I would like to extract the most realistic map of streams that I can. There is no universally recommended threshold, the threshold value depends on how much detail you want. Reference stream maps are often digitized on a certain scale, showing a more or less detailed stream network. A threshold value of 1 (cell) doesn't make sense because then streams will be everywhere. As a general rule of thumb, standard deviation of the absolute flow accumulation values seems to be a useful value and should in general provide accurate stream locations. This general rule of thumb considers what detail can be extracted from a given DEM, and not where real stream heads are located. I guess that real stream heads can only be extracted from very high res DEMs. Channel tracing is not that much of a problem, independent of the threshold used, the extracted streams should match real streams. The challenge is the proper determination of stream heads (channel initiation), which can only be done with ancillary data like precipitation and soil infiltration rate. It also depends on whether permanent streams should be extracted or permanent plus temporary (e.g. seasonal) or permanent plus temporary plus ephemeral (existing only after heavy rainfall). The -b option in grass 7.0 (beauty flat areas) applies to flat areas only, and as the name says, beautifies flat areas. Stream locations might or might not be more accurate with the -b option. The -b option applies to flat areas in the DEM, i.e. zero slope and zero tangential and profile curvature. Coastal plains are not flat areas in this sense because there should still be slight variation in elevation. The flat areas targeted with the -b option are lakes, broad rivers and sea. These areas might need to be flattened in the DEM anyway, depending on the source DEM, in order to get realistic flow accumulation and stream networks. HTH a bit, Markus M ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] r.stream.extract Montgomery
stephen sefick pisze: I would like to use the Montgomery method to extract streams (exp~2 from the paper). What should the threshold value be? And in general what should the threshold value be? I am working in the southeastern coastal plain (USA), which is characterized by low gradient (if this helps). for flat areas Montgomery's method is not a good option. Generally that method was created and tested on areas with gradient 5%. I am using 1m resolution LIDAR data. I would like to extract the most realistic map of streams that I can. Chmmm... what you mean realistic? Maybe use existing stream network will be the best solution? For coastal plains where is no real vallyes the r.watershed's treeshold with -b option seems to be best option I am also trying to track down the inttermitance perminance threshold. Any guidance would be greatly appreciated. kindest regards, ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] r.stream.extract Montgomery
What is good guidance for where to set the threshold? I am in the coastal plain, but In the very upper part ~10 miles from the piedmont, so it is not terribly low gradient (I am going to quantify this not terribly low gradient soon). I don't see the -b flag for r.watershed 6.4svn checkout (probably a week ago) in the man pages. I said realistic because you can set the threshold to 1 for a 1m res dem. There are lines that don't even look like streams all over the place. Is it possible to extract streams with the landscape (Mongomery) as a guide for the threshold? I want to be able to do this so all of the stream ordering is done on a consistent stream network among watersheds. If this is not possible then I will use a threshold and use this for all stream network extraction on all of the other dems that I need to process so they are all comparable. I appologize if this doesn't make sense, but I will explain in greater detail if I need to. kindest regards, Stephen Sefick On Thu, Aug 5, 2010 at 9:35 AM, Jarek Jasiewicz jar...@amu.edu.pl wrote: stephen sefick pisze: I would like to use the Montgomery method to extract streams (exp~2 from the paper). What should the threshold value be? And in general what should the threshold value be? I am working in the southeastern coastal plain (USA), which is characterized by low gradient (if this helps). for flat areas Montgomery's method is not a good option. Generally that method was created and tested on areas with gradient 5%. I am using 1m resolution LIDAR data. I would like to extract the most realistic map of streams that I can. Chmmm... what you mean realistic? Maybe use existing stream network will be the best solution? For coastal plains where is no real vallyes the r.watershed's treeshold with -b option seems to be best option I am also trying to track down the inttermitance perminance threshold. Any guidance would be greatly appreciated. kindest regards, -- Stephen Sefick | Auburn University | | Department of Biological Sciences | | 331 Funchess Hall | | Auburn, Alabama | | 36849 | |___| | sas0...@auburn.edu | | http://www.auburn.edu/~sas0025 | |___| Let's not spend our time and resources thinking about things that are so little or so large that all they really do for us is puff us up and make us feel like gods. We are mammals, and have not exhausted the annoying little problems of being mammals. -K. Mullis ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] r.stream.extract Montgomery
What is good guidance for where to set the threshold? Try using some already existing data for mapped streams, such as the national hydrography dataset. You can adjust the threshold of the accumulation until your modeled output is close to already mapped features. I don't see the -b flag for r.watershed 6.4svn checkout (probably a week ago) in the man pages. I think the -b option starts in version 7 (?) I said realistic because you can set the threshold to 1 for a 1m res dem. There are lines that don't even look like streams all over the place. Threshold relates to the accumulation of upgradient cells. Its usually a larger number in the thousands to 10s of thousands (at least from my data/experience). Is it possible to extract streams with the landscape (Mongomery) as a guide for the threshold? Try without altering the montgomery exponent and see how well it matches known features. Mark On Thu, Aug 5, 2010 at 9:35 AM, Jarek Jasiewicz jar...@amu.edu.pl wrote: stephen sefick pisze: I would like to use the Montgomery method to extract streams (exp~2 from the paper). What should the threshold value be? And in general what should the threshold value be? I am working in the southeastern coastal plain (USA), which is characterized by low gradient (if this helps). for flat areas Montgomery's method is not a good option. Generally that method was created and tested on areas with gradient 5%. I am using 1m resolution LIDAR data. I would like to extract the most realistic map of streams that I can. Chmmm... what you mean realistic? Maybe use existing stream network will be the best solution? For coastal plains where is no real vallyes the r.watershed's treeshold with -b option seems to be best option I am also trying to track down the inttermitance perminance threshold. Any guidance would be greatly appreciated. kindest regards, -- Stephen Sefick | Auburn University | | Department of Biological Sciences | | 331 Funchess Hall | | Auburn, Alabama | | 36849 | |___| | sas0...@auburn.edu | | http://www.auburn.edu/~sas0025 | |___| Let's not spend our time and resources thinking about things that are so little or so large that all they really do for us is puff us up and make us feel like gods. We are mammals, and have not exhausted the annoying little problems of being mammals. -K. Mullis ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] r.stream.extract Montgomery
stephen sefick pisze: What is good guidance for where to set the threshold? I am in the coastal plain, but In the very upper part ~10 miles from the piedmont, so it is not terribly low gradient (I am going to quantify this not terribly low gradient soon). I don't see the -b flag for r.watershed 6.4svn checkout (probably a week ago) in the man pages. I said realistic because you can set the threshold to 1 for a 1m res dem. There are lines that don't even look like streams all over the place. Is it possible to extract streams with the landscape (Mongomery) as a guide for the threshold? the rule for Montgomery is described in literature cited in r.stream.extract help. Look into this. In general, Montomery's assumptions take into account annual precipitation versus slope gradient * SCA (specific catchment area). It is not a problem if you have 1m dem, because in that one specific situation SCA = accumulation, so you can use a threshold = 200, as suggested by Montogmery and Dietrich for Appalachian. But it is not possible to say which value is best. I want to be able to do this so all of the stream ordering is done on a consistent stream network among watersheds. If this is not possible then I will use a threshold and use this for all stream network extraction on all of the other dems that I need to process so they are all comparable. I appologize if this doesn't make sense, but I will explain in greater detail if I need to. kindest regards, Stephen Sefick On Thu, Aug 5, 2010 at 9:35 AM, Jarek Jasiewicz jar...@amu.edu.pl wrote: stephen sefick pisze: I would like to use the Montgomery method to extract streams (exp~2 from the paper). What should the threshold value be? And in general what should the threshold value be? I am working in the southeastern coastal plain (USA), which is characterized by low gradient (if this helps). for flat areas Montgomery's method is not a good option. Generally that method was created and tested on areas with gradient 5%. I am using 1m resolution LIDAR data. I would like to extract the most realistic map of streams that I can. Chmmm... what you mean realistic? Maybe use existing stream network will be the best solution? For coastal plains where is no real vallyes the r.watershed's treeshold with -b option seems to be best option I am also trying to track down the inttermitance perminance threshold. Any guidance would be greatly appreciated. SWAT (soil water assessment tool) but not for GRASSS. kindest regards, ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
