# Tag Info

23

Altimeters use barometric pressure to measure altitude or elevation. Your watch likely uses a Baro-Altimeter. The problem with that is that the barometric pressure also changes with the weather. As the barometric pressure goes down your altimeter watch will think you are going up in altitude even though you are solidly on the ground. For such watch ...

18

First off, let me say how much I appreciate this question. I have seen so many example of inappropriate colour palettes applied to digital elevation models that it's good to see that people are thinking about this. There are some really good answers here too, but here is my opinion. I doubt that there is a universally good palette but rather a group of ...

18

Imagine several satellites spread out evenly above you. Now pick just one satellite. Visualise a sphere centered around that satellite with a radius of your exact distance from it. Do the same for every satellite in view. What you're seeing now is a bunch of spheres that intersect exactly where you're standing. That's how a GPS reading works, ...

17

I would recommend to look outside ArcGIS) Very easy using the free gdal software: http://www.gdal.org/gdaldem.html gdaldem TRI input_dem output_TRI_map Or if you'd prefer it in saga gis: http://www.saga-gis.org/saga_modules_doc/ta_morphometry/ta_morphometry_16.html

16

Let's do a little (just a little) algebra. Let x be the value in the central square; let x_i, i = 1, .., 8 index the values in the neighboring squares; and let r be the topographic ruggedness index. This recipe says r^2 equals the sum of (x_i - x)^2. Two things we can compute easily are (i) the sum of the values in the neighborhood, equal to s = Sum{ x_i }...

16

Data format I'll take it as a little exercise in how to program a data reader. Have a look at the documentation: SRTM data are distributed in two levels: SRTM1 (for the U.S. and its territories and possessions) with data sampled at one arc-second intervals in latitude and longitude, and SRTM3 (for the world) sampled at three arc-seconds. Data are ...

16

Yes, doable. Normally I'd suggest a partially transparent buffer, but I see why you want to do this cartographically. This could be slow, and you need to manually decide where you want the labels to go - but cartographically speaking, that's not a bad thing! Here's a screenshot... As you can see, no buffers. The raster underneath is unaffected. I've ...

15

My suggestions would be to utilize reverb at http://reverb.echo.nasa.gov/reverb/. First register if you have not already done so. In the search box type ASTER GDEM and Select Dataset --- note if you want a particular area this is the point where you select the box range in the map window to the left (very useful feature!). ASTER GDEM Global Digital ...

15

The elevation above the ellipsoid (ellipsoidal height) is the elevation above a mathematical model that approximates the shape of the earth. The current most common one is WGS84. These are the elevations that you'd get from a GPS. Orthometric heights are measured above the geoid or equipotential surface, that is, the surface of equal gravity. MSL is "mean ...

14

I usually come at this question from the angle of "what is going to enhance, and not obscure, my data?". Tufte talks about the some of the uses of colours in maps: to label, to measure, to represent, and to enliven. Choosing DEM colours is usually mostly for the latter (enlivening) - to make them look nice. For example, the default 'atlas coloring' of many ...

14

As a geologist, I often use this technique to make geological cross section in pure Python. I presented a complete solution in Python: Using vector and raster layers in a geological perspective, without GIS software (in French) I present here a summary in English: to show you how to extract the elevation values ​​of a DEM how to treat these values If ...

13

The 3-arc second (~90 meter) SRTM covers all of Europe to 60° N, has good positional accuracy and is part of the public domain. CGIAR provides an interface for easily downloading the data tiles, which are provided in both ASCII and GeoTIFF formats.

13

Expanding on one of themes in Simbamangu's very good answer: the basic problem with elevation shading using any colours at all other than neutral greys is the inescapable tendency for us to interpret meaning from the colours. For example a common rendering technique is to use deep greens for the valley bottoms, progressively lighten as one travels upslope, ...

13

Following on from the comments, here's a version that works with perpendicular line segments. Please use with caution as I haven't tested it thoroughly! This method is much more clunky than @whuber's answer - partly because I'm not a very good programmer, and partly because the vector processing is a bit of a faff. I hope it'll at least get you started if ...

12

You are searching Extensions > 3D Analyst > Interactive 3D analysis tools. How to create a profile graph from digitized features of a surface 1.In ArcMap, click the Layer drop-down arrow on the 3D Analyst toolbar and click the surface that you want to profile. 2.Click the Interpolate Line button Interpolate Line button. 3.Click the surface and digitize ...

11

The GRASS command r.profile performs this (documentation, source) and should provide a good basis for implementing a cross section, and is available under the GPL.

11

Maybe QGIS "Profile" plugin source code can be a start.

11

The highest elevation within 10 km is the neighborhood maximum value computed with a circular 10 km radius, so just extract a profile of this neighborhood maximum grid along the trajectory. Example Here is a hillshaded DEM with a trajectory (black line running from bottom to top): This image is approximately 17 by 10 kilometers. I chose a radius of ...

10

If you are looking to convert DEMs to contour lines: QGIS Plugin Contours For converting scanned topo maps to vector contour lines, Arcscan is one of the easiest (and most sophisticated) programs out there. However, there are several opensource alternatives, including a promising GRASS approach: Trace vector contours from a scanned map. Additional ...

9

For a local solution, GRASS can be scripted to do this: # extract raster values at our points # use cubic convolution for interpolation between DEM locations v.drape in=my_pts out=pts_srtm_elev type=point rast=srtm_dem method=cubic I ran an extended version of this for one of my use cases and performance of v.drape was no issue at all.

9

Briefly, USGS has application services (Option #1), but for some data sets it's also possible to generate direct download URLs (Option #2) to the public location of files. Download Option #1: USGS Application Services There's documentation here about the web services that are available: http://nimbex.cr.usgs.gov/app_services.php If you're interested in ...

8

This is a job for the Raster Calculator, it's under the Raster menu (don't confuse this with RasterCalc!). The expression: snowdon_dem.tif@1 + 10 will do this, where 'snowdon_dem.tif@1' is the name of the original DEM. You can also find additional informations here. Nick.

8

Here's a more programmatic way of using GDAL than @Aragon's answer. I've not tested it, but it is mostly boiler-plate code that has worked for me in the past. It relies on Numpy and GDAL bindings, but that's about it. import osgeo.gdal as gdal import osgeo.osr as osr import numpy as np from numpy import ma def maFromGDAL(filename): dataset = gdal.Open(...

8

I would highly recommend going through the Contour lines to DEM tutorial, available from GRASS wiki. Essentially, they describe different interpolation methods used to produce a DEM. Avoid IDW interpolation using contour lines (i.e. described in one segment of the tutorial) since this is an inappropriate use of the interpolation method. The GRASS module ...

8

The best all round tool here is a raster calculator. gdal_calc is a GDAL raster calculator implemented in Python here, with some examples here. If you e.g. wants to keep values above +50: gdal_calc.py -A input.tif --outfile=result.tif --calc="A*(A>50)" --NoDataValue=0 You can specify several files -A to -Z, where each of them get a corresponding ...

8

The easiest way is to edit the aplha for the fill

8

Creating watersheds should help you locate both ridges and hill top. Then, I would define a hill top as a local maximum, while a point on a ridge is not the maximum (there is one other point higher or equal to this point). You can identify local maxima using the focal statistic tool. another way to look at the problem is to analyse at the opposite of your ...

8

Defining ridges vs hill/mountain tops is pretty scale-dependent. Jeff Jenness covers conceptually how to model topographic landforms in his article Some Thoughts on Analyzing Topographic Habitat Characteristics. If you poke around on his website, you can find his poster on this as well, under ArcGIS tools > Land Facet Corridor Designer. (Link is here) Jeff ...

8

One thing to keep in mind is that lat/long is geodetic and not geocentric: If we were to calculate elevation as a radius from the center of the ellipse, our elevation lat/long would be different than our horizontal lat/long! This is why there are two different datums. The horizontal datum is just a smooth ellipse, because it's easier to do trig ...

8

Building on Michael's excellent answer, I would recommend using the Con (Spatial Analyst) tool to take a "slice" out of your DEM. The first screenshot shows the parameters you would likely want to use. The second screenshot shows the results of the Con function (as stylized MDOW hillshades) derived from the resulting DEM's.

Only top voted, non community-wiki answers of a minimum length are eligible