GML file for the Cadastre: what it is and how it works

A GML file is the file that holds the shape of a plot written in coordinates, so that the Cadastre’s software can read it, validate it and compare it with the neighbouring plots. It is what a notary’s office, the Land Registry or the Cadastre (Catastro, Spain’s land and property register for tax purposes) mean when they ask you to “provide the GML”. Here we explain what it is, what it is used for, how a surveyor delivers it and how it is opened.

What is a GML file?

GML stands for Geography Markup Language. It is a type of text file, based on XML, designed to store geographic objects: points, lines and polygons with their coordinates. A plot GML contains, in essence, the polygon of the property, that is, the ordered list of the coordinates of its corners, the coordinate system they are in and some identifying data.

It differs from the PDF plan in the most important way: people read the plan and a machine reads the GML. The plan can be very clear and be no use to the Cadastre. The GML can be perfect for the Cadastre and mean nothing to someone who looks at it as text. Normally they go together. If you want to understand the plan that accompanies the file, we have a guide to the georeferenced plan.

Why does the Cadastre ask for it?

The Cadastre uses the GML to receive, check and incorporate the geometry of a plot accurately. It is usually requested in these cases:

• When the shape or area of the plot changes: subdivision, merger, correction of boundaries or area.
• To obtain the graphic validation report (IVGA): the technician submits the GML on the Cadastre’s electronic office and it replies whether the geometry fits the neighbouring plots. We explain it in graphic validation report.
• To register in the Land Registry a property with its georeferenced graphic representation, as provided for in the Mortgage Law after the 2015 reform.
• To regularise a building when the position of the building has to be fixed (ICUC).

The GML is technical documentation. It describes the geometry, but it does not prove ownership or replace the procedure of the Cadastre, the Registry or the town council, which are the ones that decide.

What does a plot GML contain?

Although the structure is defined by the Cadastre and is based on the European INSPIRE standard, what is inside can be summed up:

• The polygon: the coordinates of all the corners, in order, with the last equal to the first so that the outline closes.
• The coordinate system: for this area, ETRS89 UTM zone 30 north (code EPSG:25830).
• Identifying data for the plot or the operation, depending on the procedure.
• If there are several plots (a subdivision, a merger), one outline for each, with no overlaps between them.

That is why a GML made from a sketch, an image or an unmeasured drawing is no use: the coordinates must come from a survey, not from drawing them by eye. The same geometry in another system appears shifted, and the Cadastre rejects it or places it somewhere else. If you need to go over this point, we explain it in survey coordinates.

How does a surveyor deliver it?

In our case the GML is a part of the commission, not a loose file:

1. We measure the plot in the field with GNSS RTK and a total station.
2. We calculate the coordinates of the corners in ETRS89 UTM 30N.
3. We generate the GML and check that the outline closes, that there are no duplicate corners or crossing lines and that it does not overlap its neighbours.
4. We deliver the GML together with the PDF plan, the DWG or DXF if you need it and a note explaining what is in each file.
5. We submit the validation on the electronic office when it is part of the commission, and give you the IVGA with its verification code.

It is signed by Vicent Doménech, geodesy and cartography engineer, chartered surveyor no. 8948. It all forms part of our Cadastre, Registry, GML and IVGA service.

In which formats is it delivered?

Each format serves a different purpose, and they are not interchangeable:

Format	What it is for
GML	Submitting the geometry to the Cadastre and the Registry
PDF	Reading the plan, signing it and submitting it on paper or on screen
DWG / DXF	The architect or the practice opens it in their CAD program
SHP / GPKG / KML	Opening it in a GIS or a viewer, for example QGIS or Google Earth

If your notary or architect needs another, tell us when you commission the job and we will prepare it.

How do you open a GML file?

Because it is a text file, any editor will show it, but you will mostly see numbers. To see the plot drawn you need a mapping program:

• QGIS, which is free: drag the file into the window and the plot appears. It is best to tell it the coordinate system, EPSG:25830.
• A Cadastre or public administration viewer, which shows the geometry over the plot map.
• A CAD program that imports GML, although not all of them do so equally reliably.

If, when you open it, the plot appears in the sea or a hundred metres from where it should be, it is almost always a coordinate system problem, not a measurement problem.

Common mistakes with the GML

• Coordinates in another system. A plan in ED50 converted without transformation appears shifted as a whole.
• Unclosed outline. The last corner differs from the first by a few millimetres.
• Duplicate corners or crossing lines, which come from exporting a CAD drawing without cleaning it.
• Overlap with a neighbouring plot, which is not always a measurement error: sometimes the Cadastre itself is shifted.
• A GML made without measuring, from a photo or a sketch.

When the result is negative, the important thing is to read the reason before touching anything. We explain it in what to do if the graphic validation report is negative.

Frequently asked questions

Can I make the GML myself?

It can be generated with free tools, but the value lies in the measured coordinates and in the file passing validation. If the geometry does not come from a survey done with the right equipment, the procedure gets stuck at validation.

Does a favourable GML update the Cadastre automatically?

No. It is technical documentation for a procedure. The update depends on the procedure and on the competent authority.

How many GML files are needed?

It depends on the procedure. In a subdivision, normally one for each resulting property and another for the original one. Each notary’s office or Registry has its own conditions, so it is best to ask before measuring.

Is the GML I download from the Cadastre any use?

The one you download from the electronic office describes the plot as it is today in the Cadastre. It cannot be used to correct it, because a correction requires a new survey.

If you have been asked for the GML of a property, tell us about it on the contact page and we will review what is needed before measuring.

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Written by Vicent Doménech, geodesy and cartography engineer, chartered surveyor no. 8948.