gpxqgis
I just got a Garmin etrex and promptly took it for a hike. I can get the path and the trackpoints into QGIS with no issue. However, I cannot figure out how to get the distance between the trackpoints. I've played with the CRS and the file formats (converting to .shp for example).
When I go into the attribute table and attempt to calculate a new field using $length, all I get are NULL values.
Any ideas?
It is very difficult to know exactly where things went wrong without knowing details about the inputs that you used for each tool. However looking at the two images that you embedded into your post, it looks to me like the common problem of running a flow path analysis on a non-hydrologically preprocessed DEM, i.e. a DEM for which you have removed all of the topographic depressions and flat areas through either depression filling or breaching. The ArcGIS tool requires an input of a D8 flow pointer raster, which clearly you provided from a depressionless DEM. I think you may have provided the r.flow tool input the original DEM which contained depressions. Take the following images as examples:
The first image is a maximum upslope flow-length grid calculated from a depressionless (breached) DEM and the second one was derived from the original DEM, which contained numerous artifact topographic depressions. You'll notice that the flow paths have been severely truncated because as flow encounters a depression or flat area, there are no downslope grid cells for the flow path to continue towards. Just as is the case with your data, the maximum flow path value in the grid is considerably less in the second raster as a result of this flow path truncation. I think you need to double-check that you input a hydrologically corrected DEM into the r.flow tool. I know from experience that this is one of the most common errors when performing DEM-based hydrological analysis. Otherwise, it is possible that you simply provided the r.flow tool the wrong input, e.g. the same D8 flow pointer that you provided the ArcGIS flow length tool (note r.flow requires a DEM and not a flow pointer as an input). Again this would be a fairly common mistake and the effect would be identical, i.e. truncated and discontinuous flow paths which is what you seem to have. I hope that helps.
Although it doesn't solve the problem with QGIS, you can, as mentioned in a comment, use DNR Garmin (DNR GPS) to convert your ESRI shapefile to GPX. This is what I do most of the time.
It is very simple. Just open the shapefile with the program, select the Id field if prompted, then export (or save as) to the desired format (in your case, GPX).
Best Answer
You need to convert it to a line, lines have lengths, points do not. You will need a plugin like Points to Paths to convert the points to lines and then you can get the length of the lines.