measureline.py

def measureline(search,reverse=False,offset=0):
    """
    To be run in the QGIS python console. Select a line, run measureline(<search>) and it will put a marker at the point 
    <search> mapunits along the line from the start of the line and print out the lat,lon of the point.
    By either setting a negative search length or setting reverse to True, it will measure from the end of the line instead. 
    A number given as <offset> will be subtracted from <search>. If an offset is given, search should be a positive value.
    
    Developed in QGIS 3.20, will probably run in other 3.X version, will probably not work in QGIS 2.X 
    """ 

    search=search-offset
    if search < 0:
        if offset==0:
            reverse=not(reverse)
            search=-1*search
        else:
            print("ugyldig avstand")
            return(None)
    layer=iface.activeLayer()
    layerCrs=layer.crs().authid()
    crsSrc=QgsCoordinateReferenceSystem(layerCrs)
    trfContext = QgsProject.instance().transformContext()
    if len(layer.selectedFeatures())==0:
        print('Må velge en linje')
        return(None)
    line = layer.selectedFeatures()[0]
    geom = line.geometry()
    totlength=geom.length()
    print(f'Total lengde: {round(totlength)}')
    if search > totlength:
        print('For lang avstand')
        print(f'{round(search-abs(totlength))} overskytende') 
        return(None)
    if reverse:
        search=totlength-search
    canvas=iface.mapCanvas()
    marker=QgsVertexMarker(canvas)
    projCrs=canvas.mapSettings().destinationCrs().authid()
    xy=QgsPointXY(line.geometry().interpolate(search).vertices().next())
    if projCrs != layerCrs:
        #The marker is drawn directly at the canvas, so it need to be in the canvas' projection
        crsProj=QgsCoordinateReferenceSystem(projCrs)
        xform=QgsCoordinateTransform(crsSrc,crsProj,trfContext)
        xy=xform.transform(xy)
    marker.setCenter(xy)
    marker.setColor(QColor(255,0, 0))
    marker.setIconSize(15)
    marker.setIconType(QgsVertexMarker.ICON_X)
    marker.setPenWidth(3)
    crsSrc=QgsCoordinateReferenceSystem(layerCrs)
    crsTrg=QgsCoordinateReferenceSystem("EPSG:4326")
    trfContext = QgsProject.instance().transformContext()
    xform=QgsCoordinateTransform(crsSrc,crsTrg,trfContext)
    latlon=xform.transform(xy)
    print(f'{round(latlon.y(),6)},{round(latlon.x(),6)}')
    return(marker)