# Tag Info

26

Most methods to spline sequences of numbers will spline polygons. The trick is to make the splines "close up" smoothly at the endpoints. To do this, "wrap" the vertices around the ends. Then spline the x- and y-coordinates separately. Here is a working example in R. It uses the default cubic spline procedure available in the basic statistics package. ...

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

In principle, you cannot do this in a unique, accurate way without reconstructing something like the original DEM. Let's see why, by doing the computation with a (difficult but realistic) example. It begins with a small portion of a 30 meter DEM, about 15 by 11 kilometers. Contours have been computed at regular intervals: this is where you begin. To ...

14

Yes, there are several options available in QGIS: Inverse Distance Weighting (IDW) Interpolation plugin - see this for a tutorial GDAL Raster plugin - to access, click Raster > Grid (Interpolation). GDAL's interpolation is more robust because you can use other interpolation algorithms (IDW, nearest neighbor, moving average, etc.). This tools only ...

13

Evaluation of the options Contour lines represent continuous surfaces, so their comparison ultimately is a proxy for comparing those surfaces. Because both the surface values (elevations) and locations are potentially subject to error, there are two components to the comparison: in terms of value and in terms of position. The two cannot be separated, ...

12

Here we go then, a method to get some control over contour label placement, see the picture. The first thing I did was to load the contours. Next, on a new line shapefile, I drew a line from the top of the hill to the bottom (the labels in the picture are aligned with this). I then buffered the line and used the buffer to clip the contour layer. After that ...

11

Cartographic rules to represent the relief as contours are presented in Imhof's famous book on relief representation, chapter C. Some of these rules are given on this wikipedia page. The main recommendation when simplifying contours it to preserve the terrain main characteristics. Smoothing the contours independently do not prevent them to overlap: It is ...

10

Assuming you have Spatial Analyst license: Interpolation: Spatial Analyst Tools -> Interpolation -> IDW ('Z value field' parameter will be your 'P' field). Reclassification of interpolated surface into 3 zones: Spatial Analyst Tools -> Reclass -> Reclassify Specify interpolated surface and ranges of desired classes: 0 - 5,... Now you have a raster which ...

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

After using v.surf.rst, it's only a question about how to display the resulting cost raster. GRASS offers r.colors to specify a color map for a raster. As far as I can see, the normal behavior of GRASS is to interpolate colors, so you you'll have to work around this: r.colors map=travel_costs color=rules 0 green 100 green 101 yellow 200 yellow 201 red ...

9

The answer depends on context: if you will be investigating only a small (bounded) number of segments, you might be able to afford a computationally expensive solution. However, it seems likely that you will want to incorporate this calculation within some kind of search for good label points. If so, it is of great advantage to have a solution that either ...

9

The Raster|Extraction|Clipper tool will help you to do this. You can open the tool and then click and drag in the raster image to select the area you want to export as a new raster (Clipping mode: Extent), and then refine the exact coordinates in the Extent fields (if necessary). Probably no way to do contours on only a selection of a raster; clip it this ...

8

GRASS has also many options http://grass.osgeo.org/wiki/Contour_lines_to_DEM available one way or another also trough the QGIS/GRASS plugin.

8

In the GRASS GIS Wiki, there is a dedicated page for this: http://grass.osgeo.org/wiki/Contour_lines_to_DEM The page demonstrates and compares a number of different methods of converting vector contour lines into raster DEM surfaces including screenshots.

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

7

The minimum contour interval is the double vertical error (RMSE or standard deviation) of the height model. You can find that for ASTER GDEM in: Lang, R. Harold, and Roy Welch. 1999. “Algorithm theoretical basis document for ASTER digital elevation models.” It's defined by the "United States National Map Accuracy Standards (NMAS)" For example: If your ...

7

I would say this depends on your knowledge of the data and how it was collected as well as how the DTM was generated from the raw data (regular grid vs interpolated from irregular points or other sources) I don't think there is a rule of thumb. Personally, I choose contour intervals based on the scale, type, content, purpose of the map, etc.

7

I use the "Buffer" option on the "Label setting" tab. (Using the labels button, not the old labels option on the layer properties dialog.) This does not wipe out the contour line, as I imagine you are wanting to do, but it does make the label legible.

7

I assume your goal is to have different stroke weights for multiples of certain values - ie, one line weight for lines that are multiples of 10M, another for multiples of 100M, and so on. The details of how to do this depend on your software, but my general approach is to open the attribute table associated with the shapefile and add a new field which will ...

7

You can specify levels using the GRASS module, r.contour.level. It's available in the processing toolbox. You can search for it when the dropdown at the bottom of the Processing Toolbox window is switched from "Simplified interface" to "Advanced interface". The "List of contour levels" parameter takes a comma separated list of elevation levels. Here is ...

6

For many contour maps a check of spatial consistency among the contour levels is not possible unless you supply additional information. Here's an example pulled arbitrarily from Google Images: The criterion for consistency is that the neighboring lines of each contour line should have values differing by at most one contour interval. But if you draw a ...

6

You first need to create a raster layer from your vector contour layer. You can use the Topo to Raster tool under the Spatial Analysis Tools>Interpolation toolbox (reference via vs 9.3.1 not sure if it is the same in 10). Once you have your raster layer, then you can use the Hillshade tool.

6

You could try to make an overlay of your contour lines with the slope grid, and only show certain contour lines depending on the average value of the slope below that contour line.

6

Sirgeo, don't try to fix up contours after they are generated, the processes take a long time and will eventually lead to intersecting contours = a nightmare! Instead smooth your raster to take the lumps and bumps out - easier maths and no topology to worry about. I find resample slightly up (cellsize x 1.5) and then back down to the original (or better) ...

6

Set environment setting extent=your DEM extent, snap raster = DEM, cell size= one for DEM. Use raster calculator expression: Con("DEM">1500,"DEM) Right click on resulting raster, Source, scroll down to see Mean

5

If you use the regular placement in maplex there are options to offset. With this result... You can set the offset units to points, mm, or mapunits. There is not a specific ladder constraint but as you notice these did ladder fairly nicely. I have seen that it is hit or miss though with out the constraint.

5

This is elevation data from the OS OpenData dataset? If you want to extract contours it's probably best to use the ASCII (*.asc) rasters. You can load these and then extract the contours by going Raster -> Extraction -> Contour. You might find it convenient to make a virtual raster (VRT) file from your nine tiles before you try extracting the contours: ...

5

Labeling using the Contour Placement style Required: ArcMap Clean (good) Data Maplex Labeling Enabled. (see http://resources.arcgis.com/en/help/main/10.1/index.html#/A_quick_tour_of_the_Labeling_toolbar/00s800000014000000/) The Maplex Label Engine groups the segments of contour features, if the symbology and label match and the features are ...

5

You have two solutions: 1) from the GRASS plugin: you are working in GRASS GIS, not in QGIS, so you need to know GRASS GIS you need to first open a LOCATION/MAPSET of GRASS (from the plugin) or create a new LOCATION/MAPSET you will then have access to the GRASS Tools to: import the shapefile in the MAPSET (v.in.ogr.qgis) transform the ...

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