Downloading TOPO

78162004a

Hello
How can download Elevation Data as Topo from File>Download online topo...? I mean Which Data Source must be chosen from "Select Data Source" menu? 

BillB

For what part of the world?

Mykle

As BillB notes, the answer(s) depend upon where in the world you are working.  

That said, you will likely have more choices with the TERRAIN DATA category, rather than POPULAR SOURCES.  There should be one or more sources for SRTM data that will cover much of the world at lower resolution (the new SRTM-Plus looks very nice from my very limited use of it).  If you are working nearer to the poles, SRTM won't cover the far north/south.  

78162004a

BillB said:

For what part of the world?

West of Afghanistan. UTM: Zone:40s WGS84

78162004a

Mykle said:

As BillB notes, the answer(s) depend upon where in the world you are working.  

That said, you will likely have more choices with the TERRAIN DATA category, rather than POPULAR SOURCES.  There should be one or more sources for SRTM data that will cover much of the world at lower resolution (the new SRTM-Plus looks very nice from my very limited use of it).  If you are working nearer to the poles, SRTM won't cover the far north/south.  

Actually what I need is Topographic map of the location I mentioned. Which one of Sources is better in this case?

Mykle

For starters (at least), use World Topo Map.  You can also download SRTM elevation data and contour it.  Don't expect a whole lot from these sources for your area of interest.  If you have other sources, they are likely to be better.  I have had reasonable results from old Russian topo maps that were available without cost, for other hard-to-find areas. 

78162004a

Mykle said:

For starters (at least), use World Topo Map.  You can also download SRTM elevation data and contour it.  Don't expect a whole lot from these sources for your area of interest.  If you have other sources, they are likely to be better.  I have had reasonable results from old Russian topo maps that were available without cost, for other hard-to-find areas. 

1- How much does World Topo Map have accuracy?   
2- What is SRTM you and how can be downloaded?

Mykle

1. World Topo Map data have limited information.  Since it doesn't cost you anything, you will need to download the coverage for your area and see if it will be useful to you.  When you have no alternatives, it may be acceptable. 

2. SRTM elevation data covers most of the world, except the poles.  Since much other data is limited in area, SRTM is good.  The SRTM v2 data has about 75m pixels, so it will be good if you are displaying a large area.  If SRTM v3 data are available, it will be more detailed (you will have to download the data to see if it includes your area).  SRTM data, as mentioned in a previous post, is available under the TERRAIN DATA category. 

Terrain data can be exported to an elevation file on your computer.  Then you can load this file, rather than download the on-line data every time you zoom or move around your project area. 

The World Topo Map data may be more complicated.  When you export a raster file, the dialog will specify a resolution that is likely to be very small.  If you keep the default values, your download will likely require a long time and may not complete.  I recommend that you select X and Y values that are equal, and are large, like 100m to begin with.  The resulting file will be small and probably not useful.  Export another file, using a smaller value.  Watch the text labels that are exported with the raster file, and this will be a guide as you choose a resolution that is acceptable.  Too small a value will result in a large file requiring a long time to export and the text will not be readable. 

Your exported files can be loaded into the program much more quickly, so you don't have to download the data more than necessary, and you won't need an internet connection to continue your work with this data.