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Digital Terrain Analysis - Science topic

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There are many accuracy measures for validating DEM such as RMSE, mean absolute error, standard deviation etc... But how can I measure the terrain gradients, is it the difference between highest and lowest point of the DEM with respect to a reference DEM? How can I measure spatial distribution of difference DEM?
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The following paper might be helpful:
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I am aware of computational costs while working with high resolution data, so this question is more of hypotetical nature and is related to the relevance of friction parameter in inundation modeling. The question is also software(model)-dependent.
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Thank you Chandan.
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It seems impossible since most of analyses are based on 3x3 cell neighborhood but I would like to hear your experiences. I know that some hydrodinamical models like TELEMAC are able to perform this.
Cheers
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Dear Mirko,
Please a look to the attached paper, which may address your question. It is about using multi-resolution image representation for texture analysis. I used MATLAB to implement the presented idea. But it is not difficult to implement it in ArcGIS. These are so many applications that can benefit from the instanteneous use of multi-resolution image representation such as image matching and the detection of the intrinsic scale of image features.
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Looking for information about two sets of questions: (1) What drivers are likely to shape future hydrographic requirements? (2) How are these drivers changing hydrographic requirements? I am interested in a comprehensive overview of trends if, i.e. technical considerations (e.g., big data, bandwidth and connectivity) are as important as future patterns in using the maritime domain (e.g., underwater exploration).
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Thank you, Hafez
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HI, all
I want to get the program to extract Digital terrain models (DTM’s) or
Digital height models (DHM’s) for a certain area from Global Gravity Field Models related to Topography For example  dV_ELL_RET2012 
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In Mathcad writing
Elevation ==
Sum, for n = 1 to 2190,
for m = 1 to n, of
Coefficient C.m Cosine Lamda + Coefficient S.m Sine Lamda
is so easy, since the proper notation is available, but in Mathcad, we can *write*
d^n / d cos^n (theta) inline but it won't expand, even with Live Symbolics *on*.
So what is needed is to write it out of line, with Live Symbolics on, Evaluate Symbolically, and Paste the result. However, even at order 5, the result may be what Mathcad calls Huge. Too big. So we shall see.
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In general, the term 'geological complexity' is used qualitatively. Are there any indicators, which can quantify the complexity? Or how the complexity of terrains be compared?
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I think you find parallels with the question in the paper of Iván Almár and coauthor: the London scale. The authors try to formulate the impact of a discovery which is complex enough (for example: to find life somewhere in the Solar System). Earlier other scales were also used by I. Almár in connection with the CETI researcg.
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Extracting the morphologic attributes (such as max slope or vertical relief) of a feature at the seabed, or onshore, is only meaningful if the digital terrain model (DTM) used have enough resolution for it. Therefore, what I am looking for is not the smallest feature that can be identified, but the smallest feature that can be characterized with a given resolution.
Consider a pockmark of 40 meters diameter, it can be seen in a dataset with a raster resolution of 10 meters. But, the mean slope value extracted at that resolution will be necessarily lower that if extracted using a 2 m resolution dataset. However, if characterizing a seamount of more than 400 m long the 10 meters resolution should provide accurate values.
Does anyone know of some kind of table defining that is the minimum feature size that can be accurately characterized for different resolutions?   
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Hi Joana,
Nyquist says we need to sample what we want to measure at twice the frequency we want to analyse (at least). So if we have a topographic process changing every 2 m, we need to sample every 1 m at least to get an accurate representation (this is best shown when plotting a sine wave: if you look at the peaks and troughs, and think about where best to sample it to find out that it is a sine wave, you need to find the peaks, the troughs and values in between).
In your case, you are using a DTM with 10-m resolution and looking at a pockmark 40 m in diameter. In the best of cases, this pockmark will cover 4 different points. The slope is the first derivative of the topography, so it uses two points. This means you can calculate the "local" slope over 3 points at most. And when you calculate the mean slope, you take the mean of 3 values only: not the most representative. So it's all a matter of statistical significance.
For a 400-m seamount, at the same resolution, you get at best 40 points covering the diameter of the seamount. This makes for 39 local slope values. How representative they are will depend on the overall shape of the seamount, i.e. before the inflexion point (second derivative, now). 
In conclusion, there is no fast and simple rule, as pointed out by previous responders. The first thing is to check how many data points cover the feature in question: is that enough to get a clear idea of its morphology? The second thing is how many points you can extract for local and average slopes, and how many points each of them is based on. Good software (like ArcGIS) allows you to define the radius or number of points over which you perform these calculations. It should never be a black box.
I hope this answers your question: we could go into more detailed mathematics to have a nice equation or a table, but it will never replace the experience of the interpreter (you :-) in deciding whether the resolution is good enough or not.
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I am introduced with methods for determining the right cell size according to sample characteristics and with visual validation of DEM resolution suitability, but is there any statistical indicator which is calculated after DEM generation (e.g. descriptive statistics), which is relevant ?
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Not sure I completely understand the question. Cell size is usually equal resolution, thus a resolution of .3m equals a .3x.3m size pixel, 1m equals a 1x1m pixel, a 5m resolution equals a 5x5m cell size, etc. Suitability is determinate on the application of the DEM. Thus to located man-made features 1m resolution or less is required, to assess the drainage or topographic features of an area can be performed using 5m to 10m resolution, while characterizing drainage patterns or topography for a large region can best be accomplished using 30m cell size.
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Suppose I need to plan the shortest road between two cities located on a hilly terrain. Maximum slope of the road is permitted up to 1 : 10. How to plan the road? Is there any known computer application, which can help on this?
Regards,
Rishi
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One way could be cost weighted distance functions (Path Analysis) in ArcGIS!
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Basically I have DEM terrain and can load it using several software. The problem is to get the texture/satellite image of the corresponding DEM file to incorporate with the terrain. (I have been using USGS (http://nationalmap.gov/) DEM files))
Thanks in advance.
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If you know the extents of your area of interest, you can encode your imagery with geospecific information. This is what you need to do to properly drape the imagery over terrain. I would recommend using GDAL (www.gdal.org) to do this. Once you have the image embedded with geospecific data (e.g. geotiff) there are several programs that will allow you to render the terrain with the imagery draped over it.
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i want pune cities dem data from 1960 to 2015 for the research purpose....
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Hi Mahesh,
You wont get the ready to use DEM of different years as you mentioned. SRTM and ASTER is available after 2000 only.
If you want to  get the older DEM, you need to use the aerial photographs to produce the DEM through the photogrammetric techniques.
The other choice is the old toposheets, which are available from the year 1915 ( i think so) and also in the time of 1960s. These data set's use can use for the generation of Elevation surfaces from contours.
These topo maps may available with SoI. you can check that.
Hope this will help.
Vijith
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Halo Members,
 I am working with with RapidEye and TSX images therefore need for one coordinate system. My SNAP processing chain has the steps:"Read - SAR-Simulation - GCP-Selection - SARSim-Terrain-Correction - LinearTodB - Write" I am however facing some challenges.
1.  Upon using the predefined CRS (EPSG:4326-WGS 84), I get the "/ by zero" error. I do not get this error when using SNAP's default coordinate system. I have tried unchecking the zero values tab on the image properties of the loaded images, but this does not seem to work
2. When trying to write the output file, I get the error "SARSim-Terrain-Correction: please select output band for terrain corrected image". On going back to the SARSim-Terrain-Correction tab, the "Apply radiometric normalization" tab is inactive
I would appreciate your contributions towards this step.
Kuria
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Thanks for the suggestion Ram. I have already done that though, hoping to get a response in the meantime
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Hi everybody,
I need to calculate the wetness index with ArcGis. I know there are a lot of different way to estimate it. I have filled and projected my digital terrain model to obtain (with the hydrology tool) the flow accumulation map. Then I have used the wetness index formula:
ln("FLOW ACC"*900/Tan"slope_in_radians").
Unfortunately I still have negative values. 
Can someone help me?
Thanks
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Hi Manuele, try with this procedure
cellsize of the raster dem = 30
fd = flowdirection(dem)
sca = flowaccumulation(fd)
slope = ( slope(dem) * 1.570796 ) / 90
tan_slp = con( slope > 0, tan(slope), 0.001 )
sca_scaled = ( sca + 1 ) * cellsize
cti = ln ( sca_scaled / tan_slp )
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What kinds of lineaments detectable in SRTM MDE?
Hobbs (1904), O`leary et al. (1976) and Strieder and Amaro (1997), identified some kinds. What is the acceptance of these types ? Do others exitst?
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I have downloaded the Bhuvan DEM of Mumbai, the elevation values are in negative even on the hilly areas.
1.Is this negative values means the whole area is in depression?
2.I want to create a text file with XYZ values of the DEM.
Request to suggest the possible solutions.
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Vanama,
That's interesting. I'm not sure why there are negative values in your DEM. If I were you, I would try to source another DEM from a different place. 
If you're looking for a DEM, the United States Geological Survey has a website that has some world-wide DEMs on it. The highest resolution for the DEM they provide for your project area is 7.5 arc seconds. If that's sufficient, you can download it here: http://earthexplorer.usgs.gov/ (it is the GMTED2010 layer on the map). 
Hope that helps,
-Andy
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How can I download SRTM data of 30 m resolution for research in remote sensing?
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you may use this internet adress. According to the this  page, you choose latitude and longitude that you want to sudy range. for example e;east longitude, n:north latitude, w:west longitude, s:south latitude folders
Parent Directory e020n40/
e020n90/
e020s10/
e060n40/
e060n90/
e060s10/
e100n40/
e100n90/
e100s10/
e140n40/
e140n90/
e140s10/
srtm30_documentation.pdf
srtm30_version_history.pdf
w020n40/
w020n90/
w020s10/
w060n40/
w060n90/
w060s10/
w100n40/
w100n90/
w100s10/
w140n40/
w140n90/
w140s10/
w180n40/
w180n90/
w180s10/
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I fail to differentiate where I can group the informal settlements, in customary or it should stay independent?. Because I find it in Urban and mostly in peri-urban area. I don't understand in creating a tree diagram for this.
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Hi,
As Sayed noted, your question is a bit unclear. However, concerning informal settlements in general, there is no universal definition for these locations/areas. What is considered informal in one location may or not include land tenure. Sayed also notes that you should first start with a definition based on location and then move on from there. Its important to note, and I'm not discouraging you in any way, that acquiring information on the locations of informal settlements is often quite challenging for a number of reasons e.g. sensitivity of locating human populations, inefficient systems in governments providing for the disbursement of such information and of course, and most population I think, no information has been collected. These areas can exist both within (e.g. India) and the outskirts (e.g. Iran) of cities. History has a large part to play here because of previous colonialism in many developing countries casted poor communities to specific locations. Of course much have changed since then, but then again, we are often still in a state of stasis well after independence in many parts of the developing world. Each community has its own individuality just in case you venture further into comparison of inter community differences. The United Nations "The Challenge of Slums" report may be worth reading if you work with informal settlements.
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I have been used several methods with SAGA GIS to explore possible clean-up ASTER GDEM V2 (30m) from anomalies and artefacts. But it is very difficult . Any suggestions about programs (saga gis or arcgis) and strategies are welcome.
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Hi!
We found that SRTM performed better than GDEM2 for tropical forest area. The resolution can be increased (e.g. 1m, 2m and 5m) by resampling technique while maintaining the accuracy (SD=9.0m-10.4m; RMSE=9.3m-10.6m).
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I am doing erosion modelling, two of the parameters needed would be C factor Map ( I'll be using MODIS time series which is 250 x 250 m) . DEM is 5 meters. How will I deal with this? Any idea/methods on how can I extract C-factor from NDVI.? Thanks in advance.
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I totally agree with previews answer. However, maybe you can try to find data for NDVI with much better resolution (check ASTER)? Then you precision will be much higher.  
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Terrain analysis through SAGA GIS opensource software.
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For terrain analysis is the best software Global Mapper but this sources is not open. The best way if you wont to use DEM is some open source GIS is should be used Q GIS is free and really good for DEM. Some good DEM for free is possible to download from USGS site.
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From the histograms of the the three DEMs, in SRTM-X there are distinct repetitive peaks. Do you have an explanation for these peaks?
Attached figure shows histograms for four different DEMs ( Photogrammetric DEM, ASTER GDEM, SRTM X DEM)
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Hi, It looks like a bi-modal histogram... This is a typical situation for a histogram of SRTM-X, SRTM-C and ASTER, when the terrain is partially under forest and partially not. One pick in the histogram represents bare land, and the another represents forested area. This is because SRTM-C/X and ASTER do not represent terrain elevations in forested areas. There is always a positive bias in forested areas!
Regarding the photogrammetric DEM: was tree heights subtracted from this DEM? Most likely not. So, this is also a DSM, and not DTM.
BUT, considering the elevations shown on your histograms, the origin of the two local picks most likely correspond to two types of mountainous terrain presented in your AOI.  See attached figure and the publication. Regards.
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My statement:
Generally, the specification of accuracy measures is based on the assumption that the errors follow a Gaussian distribution without any outlier. But most of the times, this is not the case as objects above the terrain, such as vegetation, buildings, etc. is present which gets ignored during the filtering program.
Peers comment on my statement:
Elevation errors MUST be distributed normally, otherwise many (probably all) the methods to determining the accuracy of the measurements are wrong.
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I would provide a simple counter example to the Peer reviewer's assertion. For example an image of elevation beneath the golden gate bridge....
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I am researching DEM applications, and in particular algorithms to estimate partial derivatives of height(x,y) given just sparse sample values. To test it, I will need to have analytical expressions of reasonable functions f(x,y) previously used by other researchers in published papers and/or books for DEM applications. Could anyone suggest examples, and the associated reference?
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Thank you Giulia. I acknowledge that my post maybe was not clearly enough. I am looking for analytical expressions of the height(x,y) deemed reasonable for other researchers for any purpose. I will go anyway through your references; they might have what I am looking for, and certainly will be useful if finally I ended up with a comparison paper.