how can i find the location of the Revit Project Base Point

I'm afraid the global offset is not the position of the project base point since the project base point is movable in Revit by changing its 'E/W', N/S, and Elev parameters.

Now I figured it out. We can get the project base point's position in Revit internal coordinate system by the following:

async getProjectLocationToModelTransformation(model) {
    const aecModelData = await Autodesk.Viewing.Document.getAecModelData(model.getDocumentNode());
    const refPointTransformation = this.readMatrixFromArray12(aecModelData.refPointTransformation);

    const projectLocationToModelTransformation = new SylvesterMatrix(refPointTransformation.elements).inverse().toThreeMatrix4();
    return projectLocationToModelTransformation;
}

async getBasePointData(model, category = 'Revit Base Point') {
    return new Promise(async (resolve, reject) => {
        const found = await this.searchAsync(model, category, ['Category'], { searchHidden: true });
        if (!found || found.length <= 0) return reject('Base point not found');

        const result = await this.getBulkProperties2Async(found, { propFilter: ['N/S', 'E/W', 'Elev', 'Angle to True North'] }, model);

        if (!result) return reject('Base point not found');

        const data = result[0];
        return resolve(data);
    });
}

async reportProjectBasePoint(model, useViewerSpace = false) {
    const projectLocationToModelTransformation = await this.getProjectLocationToModelTransformation(model);
    const basePointData = await this.getBasePointData(model);

    const eastWestProp = basePointData.properties.find(p => p.attributeName == 'E/W');
    const northSouthProp = basePointData.properties.find(p => p.attributeName == 'N/S');
    const elevProp = basePointData.properties.find(p => p.attributeName == 'Elev');
    const angletonProp = basePointData.properties.find(p => p.attributeName == 'Angle to True North');

    const eastWestVal = Autodesk.Viewing.Private.convertToDisplayUnits(eastWestProp.displayValue, eastWestProp.type, eastWestProp.units, model.getUnitString());
    const northSouthVal = Autodesk.Viewing.Private.convertToDisplayUnits(northSouthProp.displayValue, northSouthProp.type, northSouthProp.units, model.getUnitString());
    const elevVal = Autodesk.Viewing.Private.convertToDisplayUnits(elevProp.displayValue, elevProp.type, elevProp.units, model.getUnitString());

    const basePoint = new THREE.Vector3(eastWestVal.displayValue, northSouthVal.displayValue, elevVal.displayValue);
    const basePointInRvt = basePoint.clone().applyMatrix4(projectLocationToModelTransformation);
    if (useViewerSpace)
        return basePointInRvt.clone().applyMatrix4(model.getModelToViewerTransform());

    return basePointInRvt;
}

await reportProjectBasePoint(viewer.getAllModels()[0]); //!<<< PBP position in Revit space
await reportProjectBasePoint(viewer.getAllModels()[0], true); //!<<< PBP position in viewer space

Please find here for the full code: https://gist.github.com/yiskang/e96f544252a989daef022cd89e799f46

Here is my test result: