How to change (longitude, latitude) to QGraphicsItem(x,y) ?

[sonichy]

I have already get the point(longitude, latitude) of gpx file, how to draw them on QGrahpicsScene ?

QFile file(filePath);
if(!file.open(QFile::ReadOnly))
    return;
QDomDocument doc;
if (!doc.setContent(&file)) {
    file.close();
    return;
}
file.close();

QTreeWidgetItem *TWI_root = new QTreeWidgetItem(ui->treeWidget);
TWI_root->setText(0, QFileInfo(filePath).fileName());
TWI_root->setIcon(0, QIcon::fromTheme("application-gpx+xml", QIcon(":/icon.png")));

QDomElement root = doc.documentElement();
QDomNodeList DNL = root.elementsByTagName("trkpt");
QPainterPath PP;
for (int i=0; i<DNL.count(); i++) {
    QDomNode DN = DNL.at(i);
    if (DN.isElement()) {
        QDomElement DE = DN.toElement();
        qreal lon = DE.attribute("lon").toDouble();
        qreal lat = DE.attribute("lat").toDouble();
        QTreeWidgetItem *TWI_point = new QTreeWidgetItem(TWI_root);
        TWI_point->setText(0, QString("%1").arg(lon, 0, 'f', 14) + "," + QString("%1").arg(lat, 0, 'f', 14));
        TWI_point->setIcon(0, QIcon(":/marker.png"));
        QGraphicsEllipseItem *GEI = new QGraphicsEllipseItem(lon, lat, 5, 5);
        GEI->setFlags(QGraphicsItem::ItemIsSelectable);
        scene->addItem(GEI);
        if (i == 0)
            PP.moveTo(lon, lat);
        else
            PP.lineTo(lon, lat);
    }
}
QPen pen(Qt::blue, 1);
QGraphicsPathItem *GPI = scene->addPath(PP, pen);
GPI->setFlags(QGraphicsItem::ItemIsSelectable);

[Pl45m4]

@sonichy

Hi,

do you want to translate your lat / long value to coordinates (x, y values) of your scene?

https://stackoverflow.com/questions/14034455/translating-lat-long-to-actual-screen-x-y-coordinates-on-a-equirectangular-map

[sonichy]

@Pl45m4 The method is not fit.
The key problem is the number is too small, I find another way : Differential Amplification. The shape is correct.

QDomElement root = doc.documentElement();
QDomNodeList DNL = root.elementsByTagName("trkpt");
QPainterPath PP;
qreal lonc=0, latc=0;
for (int i=0; i<DNL.count(); i++) {
    QDomNode DN = DNL.at(i);
    if (DN.isElement()) {
        QDomElement DE = DN.toElement();
        qreal lon = DE.attribute("lon").toDouble();
        qreal lat = DE.attribute("lat").toDouble();
        if (i == 0) { // first point as reference point
            lonc = lon;  
            latc = lat;
        } else { // Differential then Amplification
            qreal x = (lon - lonc) * 10000; 
            qreal y = - (lat - latc) * 10000; // direction is opposite, add -
            PP.lineTo(x, y);
        }
    }
}
QPen pen(Qt::blue, 1);
QGraphicsPathItem *GPI = scene->addPath(PP, pen);
GPI->setFlags(QGraphicsItem::ItemIsSelectable | QGraphicsItem::ItemIsMovable);

[Kent-Dorfman]

@sonichy said in How to change (longitude, latitude) to QGraphicsItem(x,y) ?:

QGraphicsEllipseItem

correctly projecting geographic coordinates onto a flat view widget is a whole lot more complicated than you are considering. You need to consider the shape of the reference spheroid and datum standard used in taking the original readings, then it's usually easiest to deal with calculations once they have been converted from arc-degrees to UTM, thus then allowing the use of the pythagorean theorem to calculate points. See great circle algorithm. You're mapping a curved surface onto a flat projection so you have to consider orthographic distortion also....lots involved in doing it correctly...look up gdal package to automate some of it.