Linda,
the FID returned by the MVT driver, when opening on a directory, is a
composition of the (X,Y,Z) of the tile and the feature id within it.
The id part of the tile is
https://github.com/OSGeo/gdal/blob/master/ogr/ogrsf_frmts/mvt/ogrmvtdataset.cpp#L1672
:
m_nFIDBase = (static_cast<GIntBig>(nX) << m_nZ) | nY;
and then the feature ID is computed at
https://github.com/OSGeo/gdal/blob/master/ogr/ogrsf_frmts/mvt/ogrmvtdataset.cpp#L1873:
poFeature->SetFID(m_nFIDBase +
(poUnderlyingFeature->GetFID() << (2 *
m_nZ)));
so the ID in the tile is
(GetFID() - m_nFIDBase) >> (2 * m_nZ)
But I'm realizing that the (X,Y,Z) of the tile when reading the
directory are not accessible
Maybe you best chance is to slightly modify the driver so it creates a
field with that information
Even
Le 23/11/2025 à 13:01, Linda Kladivová via gdal-dev a écrit :
Hi,
I’d like to ask about one specific aspect of the MVT driver. In my
workflow, I need to track the FIDs of features inside an MVT dataset
together with their corresponding internal codes (actual parcel IDs).
My use case involves an MVT dataset of about 20 million features that
is frequently updated. For this purpose, I have implemented an
extension of the MVT driver that can open an MVT dataset in update
mode and create/delete features based on changes detected by a
preceding ETL process. Updates typically affect only a few individual
features every hour.
After the initial loading of the full dataset using |ogr2ogr| I need a
way to iterate through the dataset and build a separate table mapping
each internal FID to the parcel’s real ID. I have tried opening and
inspecting the |temp.db| file (which I do not delete):
GDALDataset* poTempDb =
(GDALDataset*) GDALOpenEx(dstTempDb.c_str(),
GDAL_OF_VECTOR, nullptr, nullptr, nullptr);
OGRLayer* poTempLayer = poTempDb->GetLayerByName("temp");
poTempLayer->ResetReading();
OGRFeature* poFeat = nullptr;
while ((poFeat = poTempLayer->GetNextFeature()) != nullptr)
{
GIntBig fid = poFeat->GetFID();
std::cout << "FID=" << fid << " — ";
int idx = poFeat->GetFieldIndex("feature");
int blobSize = 0;
const GByte* pBlob = poFeat->GetFieldAsBinary(idx, &blobSize);
std::cout << " Blob size=" << blobSize << std::endl;
}
I can successfully access the |feature| blob, but I need to decode it
in order to extract my custom parcel ID attribute and store those
values externally. This would allow me to keep track of which parcel
corresponds to which internal FID.
This is a one-time (potentially slow) process. Afterward, during
incremental updates, I will be calling |CreateFeature| and
|DeleteFeature| (and updating my external mapping table accordingly).
For edits, I currently use a combination of |DeleteFeature| and
|CreateFeature| (I haven’t implemented |SetFeature| yet).
My question is:
Is there a way to decode this blob using existing GDAL/MVT
functionality, or would this require implementing a new function?
Thank you very much for your help.
Linda Karlovská
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