Firstly I've got to say that generally I find the landranger maps to be pretty accurate. However, in some places, they can be a little off. This is probably best illustrated below:
The route shown was uploaded from a GPS unit - I was riding on the road at all times. The left hand map is the OS Landranger map. You can see that the route is a little off - particularly near the top left corner. The middle map shows the same route displayed on the Google map. Accuracy pretty good. The right hand map shows the OS Street View map. The accuracy of these are very good as well.
I really don't like to critisise the Landranger maps. Firstly I had to search hard to find an example this inaccurate - mostly they're pretty good. Secondly I think the level of detail they offer more than makes up for a little inaccuracy here and there.
It's been pointed out to me that some degree of positional inaccuracy is an accepted part of producing smaller scale maps due to the fact that it becomes increasingly difficult to accurately represent features as the scale decreases. It's easy to believe the Landranger map as shown in the LH map above is a larger scale than it really is because it is a zoomed in view - it is still a 1:50000 map.
Further to this another user has pointed out the following:
OS 1:50k maps are deliberately distorted in places, in the interests of clarity. Roads can be drawn up to 50m away from their true positions.
As they are raster maps, a bit of road is always the same width in ground metres, regardless of the display scale, and the drawn width will be considerably more than the true width. As examples, a single carriageway red A road is drawn at a scale width of about 40m, when it's actually only 7m, and a 4-5m wide white residential road will be drawn at a scale width of about 30m.
If you get several features close together, all drawn larger than their true size, something has to give. Eg a residential road sandwiched between an A road and a railway line. The choice is to either miss something out, draw things smaller, or to draw standard sized features in the wrong place.
Google maps etc are vector maps. There is a single accurate position for the centre of the road, and that's where the road is displayed. How the roads are displayed depends on the scale at which the data is being drawn. It may be decided not to show some of the data at all, so you don't see residential roads when you are looking at a whole city for example.
Nevertheless this raises a good point about all courses. Courses created from uploaded GPS traces will always be more accurate than mapped courses. If you are using a mapped course in your GPS unit then be prepared for a little inaccuracy.
The OS maps are provided free of charge by the Ordnance Survey Agency. They impose a restriction on the number of map tiles they provide per day. Once this restriction is reached there will be no OS map displayed. Come back tomorrow.
The elevation data is provided by the GeoNames web service. If for any reason this web service is unavailable then no elevation data will be shown. If this web service is slow to respond then the elevation updates will be slow.
If the supply of elevation data dries up during the creation of a course, the last good elevation received is used as the current elevation. This leads to the flat patches shown below.
There is now an elevation data repair script. This runs overnight and attempts to identify any routes with missing or corrupt elevation data. For each route identified it will re-query the elevation data and insert it into the database. For example, the corrupt elevation trace shown above was corrected to the trace shown below.
So - if your elevation data goes bad while creating a course, check again the following day. It should be corrected. If it is not, it is possible that the script has missed it. Drop me an and I'll make sure it gets corrected.
You can now edit your own courses. If you load one of your own courses (i.e. a course that you saved while you were logged on), make some changes, then save it, you will overwrite the original course.
If you have courses in the database that were saved before the log on facility was available (and therefore have no owner), send me an email with your user name and the IDs of the courses and I will associate you with these courses.
If you want to delete a course - please email me.
If you convert a tracklog file to GPX using GPSBabel, and then upload the GPX file to this site, you may get a corrupt looking route, as shown in the left hand image below.
The correct route is shown on the right; this has been uploaded directly as a trl file.
The problem is that GPSBabel (versions 1.3.4 and 1.3.5 beta) seems to assume that the trackpoints in the trl file are ordered sequentially. This is not necessarily the case. There is a further layer of processing required to stitch together the trackpoints in the correct sequence. Therefore, if you want to upload a tracklog file, please upload the raw tracklog file - don't convert to GPX first.
The route created in Follow Road mode comes from Google. They return the fastest route for the journey via car. Therefore it will try to direct you along motorways and A roads if possible. The "avoid highways" option you may have seen on Google maps is not available via the API.
Your options are to either create your route in smaller chunks - and therefore force the route to follow the roads you want, or to use drag edit mode to drag your route back off the motorways.
As you might suspect, Google knows nothing about byways, bridleways and footpaths so Follow Road mode will never create you a route along one of these tracks.