Tracklogs are the raw material of the maps. (If you don't know anything about tracklogs go and take a look at "Help us!" menu.)
There are at least three things that have influence on the usability of recorded tracklogs:
We need as accurate tracklogs as possible. You should know that tracklogs can be very different in quality depending on the following things:
- the type of the GPS receiver
In general it can be said that older receivers produce worse tracklogs and newer receivers give better quality.
There are two category of GPS receivers:
- handhelds, vehicle navigation systems
- a "computer-like" equipment (PDA for example, or even a laptop in cars!) and an "external" GPS receiver (we treat the built-in receiver as external as well! this is because there's no difference between the bluetooth and built-in receiver for recording softwares)
Handhelds are often Garmin appliances (Garmin is the most widespread brand), while external GPS receivers are various.
We're trying to categorize them from the aspect of tracklog-recording quality, from worse to better (note that this order is based on our experience):
- old etrex, gecko handhelds with gray display: they produce usable tracklog only if other circumstances are ideal (rare)
- etrex handhelds with colored display but without "x" letter in their exact type: due to a better antenna they often produce good tracklogs if you place/position them well
- etrex handhelds with colored display with "x" letter in their exact type: a more sensitive antenna and possible the amplification part is improved as well, mostly good quality tracklogs
- GPS and GPSMAP series without "x" letter in their exact type: almost the previous category, they have a little more sensitive antenna
- GPSMAP series with "x" in their exact type: these receivers have the SIRFIII chipset inside, they are the best choices nowadays
External receivers: it is difficult to give quality order due to various chipsets built in in these receivers
- "old" sirfII receivers with "XTrac" function ON: XTrac may produce confusing tracklogs, use these receivers with XTrac OFF
- "old" sirfII receivers: on adequate postioning they can produce good tracklogs
- Nemerix receivers: nemerix updates the chip version frequently, they ship more and more sensitive chipsets, quality is almost the same as SIRFIII
- SIRFIII receivers: best choice nowadays, mostly good tracklogs (but note that quality may differ depending on the brand and the version of the built-in software)
Magellan handhelds: we have very little experience with these receivers but it seems that they often produce misleading tracklogs.
PNA-s are mostly low-cost solutions, they are able to record tracklogs very rare, and they often have not too good antenna and receiver chipset. We do not recommend buying/using PNA-s. (PNA-s have another disadvantage: you are not able to upload maps to these devices.)
- the positioning/placement of the antenna of the GPS receiver
GPS system works with microwaves therefore the received signal strength depends on the orientation of the receiver's antenna.
- Garmin GPS60 and GPSMAP series has so-called Quad-helix built-in antenna that produces best signal strength when you hold the handheld (the antenna) vertically
- Other gps receivers (Garmin extrex series, most bluetooth-devices, GPS-mouses) have so-called "patch" antenna that works good on horizontal placement.
Most of the GPS receivers have the ability to connect an external antenna to it. Usage of external antennas is recommended (but not obligatory). There are three causes to use external antenna:
- you can can place it into the good orientation more easily than the GPS receiver itself
- external antennas often have amplifiers so the GPS equipment gets stronger singnals
- you can improve tracklog quality a lot with it
Commercial external antennas are almost always patch antennas with magnetic underlay, it is easy to place it on the roof of a car, for example. They cost very low price compared to the receiver itself.
- satellite geometry (the satellite positions on the sky relative to each other)
Even having the best equipment you wouldn't be able record good tracklogs if satellite geometry is bad. All receivers/softwares give a screen where you can check the positions of the satellites.
Which are the criterias of a good geometry?
-many visible satellites: if your receiver sees and uses at least 6-7 satellites then tracklogging is not in trouble, but the ideal is 10-12 satellites. With 3-4 satellites you will be very rare able to record usable tracklog.
-the satellites are not flying near each other: the best is when the satellites are distributed evenly on the hemisphere of the visible sky. Tracklog won't be good even your receiver sees 6 satellites but they are in line on your receiver's screen.
As the satellites fly over the sky the geometry changes, a quarter may be enough for a big change.
You should check the satellites screen regularly. In case of more than 10 m accuracy take a break if you can while geometry of satellites gets better.
- coverage (objects between the receiver's antenna and the satellite)
The satellites fly over your head with altitude of ~20km, and transmit radio signals with only ~50W. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains or even bodyparts! Thus, if these objects shade your receiver it cannot be able use the signals from shaded satellites.
Therefore it is important to place the receiver (or the antenna) such a place where you or your vehicle does not shade it (GPSr on shoulders, external antenna tied upon your hat, or magneted on the roof of the car).
Reflected signals can degrade the tracklog quality as well. Your receiver often hear reflected signals between or near big and flat surfaces, eg. between tall houses or near big rocks. Unfortunately you can't do too much to help the receiver avoid this problem. The only solution is going these roads/trails several times to create many tracklogs.
- number of tracklogs
The inaccuracy of tracklogs can be worked out using more than one tracklogs. In general, we get them from different people, recorded in different time, etc.
Fortunately, at the same time two GPSrs side by side will not miscalculate the position the same way! This means that if you carried two GPSr on one trip, it gives almost the same result if you had walked/drived that road twice! Hence you should take more GPSr for tracklogging if you have the possibility.
Various receivers has a lot of setable paramters this way we can't discuss every model, we're just trying to give a guideline for the settings. (And below you can find a little more detailed setting description related to the most widespread Garmin receivers.)
The main influential setting is the point frequency in the tracklog.
Commercial GPS receivers (externals and handhelds as well) refresh the current position in every seconds. This means that there's no way to set the time interval less than 1 seconds. Similarly, if you set the frequency based on real distance, eg to X meters and you go faster than X meter/sec then the trackpoints distance will be greater than X.
External GPS receivers (remember, we treat a PDA + built in GPSr as external!) almost always produces so-called NMEA data-flow. NMEA - by default - contains the position in every seconds. (NMEA also contains information about the actual satellite geometry, this can help a lot for a map editor to determine the quality of the tracklog.) NMEA files can be big, an one hour log is slightly under 1MB.
It is perfect to record NMEA data to a file eg. on a memory card, you don't have to set any options in this case. The only thing you should worry about is the memory card capacity.
Not all navigation software is able to record NMEA data-flow. If you use such a one you need additional software. For example, VisualGPSce is a small and smart program for PDAs with NMEA recording feature.
Handhelds with recording capability almost always have the ability to set distance between two track points to be recorded. "Distance" can mean the real length of the distance between the two points in meters. Or it can mean a time interval (in secs) between points.
It is most desirable to set these intervals as little as possible.
In general, it is good to set the frequency
- to 2 - 3 seconds, or 2 - 4m if you go by feet
- to 1 second or 5 meter if you go faster than a pedestrian (by car, motorcycle or even bicycle). Or set it to the least possible value if your receiver does not allow to set such little values.
A common problem is that handhelds may not have enough tracklog memory.
First you should find out the the size of your receiver's memory. Then you can calculate the log capability of it in hours or in kilometers.
For example, your receiver has 10,000 point size tracklog memory and you're about to go somewhere by car.
1) You set the point frequency to 1 sec. You'll have 10000/3600 = ~ 2 hours and 45 min only! With 50km/h the trackpoints distance will be ~14m. This is suitable for paved roads.
2) Setting the frequency to 1 meter means that the receiver tries to memorize its position after every 1m movement. If you go faster than 3,6 m/sec (you'll do by car!) then the result will be the same as with the 1sec setting (because receiver does not give position more frequently than 1/sec, as you read above). This way the 1m setting gives you larger tracklogging capacity only if you often stand in traffic jam, for example.
What if you go by feet on a trail?
1) Assume you set point frequency to 3 meters. If your speed is under ~12km/h (it will be by feet!) point distances will be 3m, this is perfect for map editing. And you can log a 30km track!
2) 2 seconds will produce ~2.2m distances if you walk with 4km/h. This is good but even if you stand for relax GPSr continues logging and consuming tracklog memory needlessly. Without relaxing this method gives ~22km log capacity.
For summation it can be said
- with external receivers record the raw NMEA data
- if you haven't got tracklog memory limitation set the frequency to 1 sec and forget these problems
- set frequency to 3-4 meters if you go by feet, car, bicycle, etc. and you know that tracklog memory will be enough
- otherwise you should calculate the least meter/second value as far as possible. Note that ~10m for trails and ~25m for roads, ~40m for highways are around the upper limit for using tracklogs for creating maps.
Garmin specific notes:
- Map-capable Garmins have a feature that places your position onto the nearest map line (road, street, etc). This so-called "Lock on road" function must be turned off to record usable tracklogs!
- Garmin x series are able to record to a memory card, in this case you can choose 1 sec recording (but don't forget to switch on the "Log to memory card" feature!)
- Garmin GPSrs have the ability to save the track. Unfortunately, the saved track can be a maximum of 500 points. If you've recorded 10,000 points and save it the GPSr's "backup" memory the log will be reduced to 500 points! This has the "side effect" that the trackpoint distances will be 50-500m what is unusable for map editing! Never use this saving feature! Record ACTIVE LOGs only (or log to memory card).
- Automatic/most frequent recording is sufficient on eTrexes with black-and-white display and Geko machines.
- If you use metric system the least setable distance-based frequency is 0.01km = 10m! Anyway, how to set distance shorter than 10 meters? You must set Units to Yard-system, then tracking interval to 3-4 yard. If you set back measurement units to metric (don't get frightened!) 0.00 km distance will be displayed but ~3-4m will be memorized!
Beside the tracklogs we need some kind of additional information to create correct maps. They can be given using waypoints.
- Points Of Interests (POIs):Not just the roads (map lines) give you helpful informations on the go. You look for specific places (points) very often: you want to entertain and you want to go to cinema, when you hungry you may look for a snack bar, or you want to oversee the mountains you hiking among then a lookout tower would be ideal. These places are represented by points on the maps and they are searchable on the most of the GPSr equipments.
You should include such points in your tracklog-files taking high care in naming. First, type the proper name if the points has one. Then append the point type after its name. You can see the usable set of point types under the "Legend" menu.
- road/path types:
Map editors need to know the type of the road you tracklogged. You can see the possible types on the "Legend" page. We know that not all the road types can be covered by this type set, it can be more detailed. But in other viewpoint the type set has too many entry and it needs too much work to determine and select the adequate type. We think this set is the trade-off between the two opinion.
The easiest way to give type-information to map editors is placing waypoins at every changing of road types and naming these waypoints after the following road types. For example, you place a waypoint named "residential street" at the start of the tracklog. And where the pavement has worsed to gravel you place a waypoint named "macadam-like"; etc.
- crossing waypoints:Due to the inaccuracy of the GPSr equipments the bunches of tracklogs of two crossing road may not cross each other in one point (this is true increasedly on trails where the paths are nor straight nor squarely). Therefore it is highly recommended to mark the middle of all the crossings with a waypoint.
- road stubs:
Until the maps don't contain all the roads, paths, etc (in short: forever :) there will be crossings where the either of the crossing road is known by a tracklog and the other is not tracklogged by anyone. In such crossings you may not be able to decide which road you need to go along if the crossing paths stand in acute angle.
This is why we encourage you to mark all the crossings with a waypoint and give them a name that tells the map editor the approximated direction of the unknown road/path. For example, you may use clock-directions (north is the 12!) in the names of waypoints: "2,11" could mean that there are two unknown paths starting from the waypoint's position, one to 2 hours direction and another to 11 hours direction (of course, do not include the clock-directions of the paths you've gone into and out from the crossing!). According to this information the map editor will insert a short leg (with "road stub" type) that guides the users of the map to the desired direction.
|accuracy||place the GPSr (or the antenna) as high as possible!|
|take care of GPSr (or the antenna) direction!|
|use external antenna if possible!|
|check the satellite geometry frequently! (and if it is bad then wait for good geometry if you can)|
|use more than one GPSr if you can!|
|GPSr settings||use the highest possible point frequency - considering the planned trip length (3m is a good choice in general)|
|record raw NMEA data if you use PDA-like equipment!|
|switch off "Lock on road" feature if you have map capable Garmin equipment!|
|log to memory card if you have "x series" Garmin receiver!|
|additional informations||mark Points Of Interest with waypoints and name them accurately!|
|mark the road types with waypoints!|
|mark the middle of every crossing with waypoints and name them according to the clock-direction convention!|