The best way would be to use digital 3D-photogrammetry. Such equipment is often available in geographic faculties of the universities, sometimes it is also used in machine engineering research or in civil engineering labs

Alternatively you can do it manually by taking two photographs and analyze the vanishing point projection and the measurements of reachable reference points, as I've done it for the Church St. Lucas in Dresden, Germany (see link - it's German document but the drawings speak for itself). However, for that you need  you need knowledge of descriptive geometry.

http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-90599

Dear colleagues

Thank you very much for your intellectual contribution. I will go through the stuff you have suggested.

regards

Asif Ali

Dear Regine

Thank you for response. Your document seems quite valuable for my research, but it is a German document. Can you kindly suggest some more references for the vanishing point projection methods.

regards

Dear Asif:

I agree with Napoleon and Regine. But one way to determine individually these arcs is certainly the new technological tools for 3D scanning. Among these there are two types:

1. Structured White Light Scanner

2. Laser Scanner total stations)

Depending on the accuracy and the size of the object to digitize used one or the other.

Best regards

Juan Pérez

Dear Asif:

You may follow the following steps:

1. Calibrate your digital camera.

2. If your arch is texture-less, put marks or project points patterns by an LCD projector on the arch. If there is enough texture on the arch go to step 3.

3. Collect overlapping image that cover your arch.

4. Measure a known distance on the arch.

5. Use standard close range photogrammetry software such as PhotoModeler for 3D reconstruction of your arch.

6. With PhotoModeler you can measure (X,Y,Z) and use them to fit a 3D model of the arch. This fitting is the 3D reconstruction of your arch. This fitting depends of the complexity of your arch. It could be a fitting for one piece or multiple pieces.

7. PhotoModeler has automatic tools for direct 3D modeling but it may not address your task, which depends one again on the geometrical complexity of your object (here: arch).

8. Indeed, laser scanner will give you a direct measurement for 3D points on the surface of the arch but you be left with the 3D modeling of your arch.

You may send me 2 to 3 overlapping images to give you more informative feedback.

Regards,

Gamal

Dear Asif,

I think my research will be useful. The arches are sized by the side-diagonal dual scale system, like the rest of the building.

Regards