While drawing routes, you can make the route snap to roads and
trails on the map, or to your existing routes or recorded tracks:
The benefit of this is that you can quickly get an accurate route,
with accurate length and climb statistics. To use this feature:
- Tap on the map at your starting point, on a road or trail.
- Tap .
Draw
- Choose a file, and optionally give the route a name, style, etc. in the Draw New Route form.
- Tap .
- Tap in the Draw panel at the bottom of the screen.
Options
- Tap .
Snap
- There are various snap options that you can set; these are described below.
- Draw by moving the pencil. The route will snap to trails and roads.
For basics of drawing routes, see the description
of the draw button.
Sources of the Snap Data
It’s important to understand that the app cannot see where trails and roads
are actually marked on the maps. The maps that this app shows are all essentially
photos; some are literally scans of paper maps, and the others are handled in
the same way.
Instead, the app has separate data that it uses only for the snap function.
For the US, this comes from the USGS
National Transportation Dataset. This ought to be the same as the trails
and roads shown on the US Topo maps, though I have found some differences.
Differences when compared to the traditional topos can be quite substantial,
due I guess either to re-routed trails or inaccurate surveying.
For Canada it uses the CanVec
Transport Networks data from Natural Resources Canada. I’ve not really
investigated how well this aligns with the maps.
As a result, the pencil may snap precisely to the trail on the map, or it may
snap to an invisible trail a short distance away, or in some cases it may not
find anything to snap to at all.
Snap Data Downloading
Snap data is normally obtained when you install maps; it’s a small additional
download. If you installed maps before this feature was added to the app in
version 2.11, then the snap data will be downloaded when it is first needed for
an area. This of course needs an internet connection. You may see a download
progress bar appear briefly when this happens.
Snap Options and Modes
The snap options offer various possible things to snap to. The first group,
map features, are the downloaded data described above. You can separately
enable trails, 4WD routes and roads.
You can also snap to any currently-visible search markers, and to your
other routes, tracks and waypoints. Be aware that routing between these and
the map features will require that shortcuts are enabled, see below.
There are six different combinations of drawing and snap modes. The drawing mode
can be Smooth or Point-to-Point; snap can be off or on, and when on the mode
can be Snap Only or Snap and Route.
Smooth Modes
We can compare the three “Smooth” modes by looking at a winding section
of trail, zoomed in:
The yellow route was drawn with snap off. It’s purely freehand, with no
reference to the actual trail beyond my attempt to roughly follow it.
The green route was drawn with snap on, in Snap Only mode. The pencil snaps
to the nearest point on the trail as it is dragged along, and draws straight
lines between these points. Note that it has missed many of the switchbacks.
The purple route was drawn with snap on, in Snap and Route mode. Here, as the
pencil is dragged along, it find the closest point on the trail, and then finds
a route along the trail to this new point. This is the mode in use in the video
at the top of this page.
Point-to-Point Modes
The differences between the snap options in point-to-point mode are best
demonstrated in an urban area with plenty of roads (but do see the warning
below about snapping to roads).
The first video above shows point-to-point mode with snap off: the pencil
moves without reference to the roads on the map and points can be placed at
arbitrary positions.
Snap Only mode, shown in the second video, does not appear to be much
different. The pencil snaps to roads, but the route segments remain stright
lines between those points.
Snap and Route mode, shown in the third video, is rather different. Now the
app computes a route from the last point to the current pencil position, and
constantly updates it as you drag the pencil across the map.
For comparison, the final video shows Smooth Snap and Route mode in the
same location. It may look similar to the Point-to-point Snap and Route
mode, but observe that it does not constantly recompute the route as you
drag the pencil.
Gaps and Shortcuts
Something to be aware of is that the snap data contains small gaps, which
prevent routing in Snap and Route mode. The app’s solution to this is to
allow shortcuts that jump over these gaps.
There are various reasons why gaps could be present. One common case is
at trailheads, where the start of the trail is separated from the end
of the road by the width of the parking lot. Clearly you can walk from
the road to the trail, but the app just sees a gap. There are also places
where tiny gaps seem to be purely discontinuities in the data being used;
this often seems to happen at administrative (e.g. National Park) boundaries.
Beware that the app can’t tell the difference between easily traversable
gaps like those and, for example, a missing bridge across a deep canyon!
You can tell the app that it’s OK to shortcut over gaps using the “Maximum
shortcut” slider in the snap options. Setting this to zero turns off
shortcuts, and the app will refuse to route over gaps. Consider setting it
to a small distance, maybe ten feet or so, for normal use - but note that
the default is zero, because of the missing bridge possibility.
Any gaps that have been shortcut are briefly indicated by a warning triangle,
and your phone may buzz as a warning that this has happened.
Note that the algorithm may not necessarily find the best place to put
the shortcut. Note also that in Point-to-point mode it will only insert
a single shortcut for each segment of the route.
Warning about Roads
Please be cautious about snapping to roads. This is not intended for
planning routes for driving! In particular:
- The app knows nothing about one-way streets, nor about which side of a divided highway to use, nor about ramps. It will wrongly let you turn from one road to another where they cross on a bridge.
- Although the app tries to distinguish between roads and 4WD routes, I doubt that this distinction is accurate in many cases.
- Roads and 4WD routes (and trails, for that matter) could be private.
- The offered routes are simply the shortest distance, making no allowance for the type of road.