[Qgis-user] GPX data in QGIS.

[Bernd Vogelgesang]

On 02.11.20 13:57, Fernando M. Roxo da Motta wrote:
> On Sun, 1 Nov 2020 17:26:46 -0500, Garth Fletcher <garth at jacqcad.com>
> wrote:
>
>    Just a side note.
>
>    If I understand it correct, this procedure will store in attribute
> table (as a temporary or permanent attribute) the coordinates as they
> are at the time it is stored.   If a point is moved through edition that
> value will not be updated automatically.  It will be necessary to store
> the new value in the table with the described procedure.

Yes and no: Since a while, it is possible to update values based on such
a function after doing some configurations.

First create your x and y fields and apply the $x and $y functions on
them through the field calculator.

When you now want them to automatically update on changes, go to

Layer Properties -> Attributes Form -> Pick the field in question ->
Section "Default Values" -> enter the function into the "Default value"
and tick the "Apply default value on update"

Done ;)

>
>> My experience is with .gpx files generated by a Garmin eTrex-20.
>>
>> When I use QGIS's Data Source Manager to open a and Add a .gxp file I
>> see several different layers:
>>    Tracks
>>    Track Points
>>    Routes
>>    Route Points
>>    Waypoints
>>
>> The one you want is "Track Points" because it contains all the
>> individual points which collectively make up the tracks. "Tracks"
>> only contains link(s) to the various tracks that have been recorded -
>> it usually only contains one item unless you have saved multiple
>> tracks.
>>
>> You may also be interested in "Waypoints" if you've recorded specific
>> waypoints on your GPS.
>>
>> So Add the layers of interest (Track Points and maybe Waypoints)
>>
>> Because a .gpx file is "input only", your first step needs to be to
>> to right-click on the layer, select Export > Save Features As...
>>
>> This will let you save the layer in a form that can be edited. Keep
>> this new layer (you can remove the original .gpx layer)
>>
>> The layer's Attribute Table contains fields for all sorts of
>> attributes, but NOT including the location (Lat,Long) data which is
>> stored separately.
>>
>> So to be able to make your own location based calculations you want to
>> copy the point locations into new fields in the Attribute Table.
>>
>> With the layer selected, select the Field Calculator (Abaccus icon in
>> toolbar)
>>    check Create a new field
>>    enter a name, e.g., "Latid"
>>    set Output field type to Decimal number (real)
>>    set Output field length to 10, Precision to 6
>>    type "$y" in the Expression window below
>>      {$y is the location's Y axis, i.e., its latitude}
>>    click OK
>> the first time it may warn you that the layer is not in edit mode and
>> than proceding will turn on edit mode - this is fine
>>
>> Now repeat for a 2nd field, using a different field name and
>> expression "Longid" and $x
>>
>> Now when you open the layer's Attribute Table you will see 2 new
>> fields at the right which contain the point's Latid and Longid
>> values...
>>
>> Save the layer if you want to keep these additions.
>>
>>
>> On 11/1/20 2:17 PM, J. M wrote:
>>> Hi again Garth,
>>>
>>> Well I parked this problem and only just came back to it. I may
>>> have missed something, but when I create the new fields you
>>> suggested in the Track they contain no information. Is this a very
>>> advanced procedure? It seems like something quite basic, but
>>> perhaps I've underestimated it (as I usually do with QGIS!).
>>>
>>> Kind regards,
>>> Jimi.
>>>
>>> On Fri, Sep 11, 2020 at 11:51 AM J. M <jimimckay at gmail.com
>>> <mailto:jimimckay at gmail.com>> wrote:
>>>
>>>      Hi Garth,
>>>
>>>      Thanks a lot for the very complete answer! I'll give it a go as
>>> soon as I resolve an unrelated Ubuntu problem. I hadn't realised
>>> that GPX data wasn't automatically visible in Properties, and that
>>> fields had to be created.
>>>
>>>      Kind regards,
>>>      Jimi.
>>>
>>>      On Fri, Sep 11, 2020 at 4:26 AM Garth Fletcher
>>> <garth at jacqcad.com <mailto:garth at jacqcad.com>> wrote:
>>>
>>>          On 10 Sep 2020 at 20:12, J. M <jimimckay at gmail.com
>>>          <mailto:jimimckay at gmail.com>> wrote:
>>>           > I can successfully import .gpx files into QGIS, and they
>>>           > are
>>>          visible and
>>>           > can be clustered in new groups (waypoints, tracks and
>>>           > routes
>>>          appear as
>>>           > independent layers). The problem I'm having is that I
>>>           > can't
>>>          seem to find
>>>           > any way to access the data about the routes (distance,
>>>           > time,
>>>          etc) which is
>>>           > clearly visible in the app I used to create the tracks
>>>
>>>          The GPX file contains a number of fields whose contents you
>>> can see /
>>>          access in the layer's Attribute Table
>>>             right-click on layer and choose Open Attribute Table
>>>
>>>          However, these are only the extra fields in the GPX records
>>> but NOT including the x,y coordinates (Long,Lat)
>>>
>>>          To access the later, use the Field Calculator
>>>             select the layer
>>>             click on Open Field Calculator icon (Abaccus) in the
>>> menu bar which is next to the Open Attribute Table icon in menu, or
>>>              in Processing > GDAL > Vector Table > Field Calculator
>>>
>>>          create new fields, called Lat and Long, as decimal numbers
>>>          and enter the expression $X or $Y (see Geometry > for
>>> details) When you click OK, the new fields will be appended and
>>> filled in with the right values.
>>>
>>>          You can then access them in the layer's Attribute Table.
>>>
>>>          With those 2 extra fields you essentially have access to
>>> everything that was recorded in the GPX file.
>>>
>>>          You can also use Field Calculator to add more fields and
>>> populate them with calculation results.
>>>
>>>          As an aside, Field Calculator cannot create new fields in
>>> the GPX file; instead it creates them as temporary virtual fields.
>>>
>>>          A better approach is to first save the data in a shape file
>>>             right-click on layer, choose Export > Save selected
>>> features then open that file instead of the original GPX.
>>>          Field Calculator will add new fields to that format.
>>>
>>>          Cordially,
>>>          --
>>>          Garth Fletcher
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>    Roxo
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