What is a Datum Target : Datum target definition, symbol and types

We already know that datum is the most important thing in GD & T. Without datum, it is next to impossible to use GD & T. But establishing datum on an irregular surface is a big challenge. The datum target helps to establish a datum on those kinds of irregular surfaces. Let’s learn more about the datum target in this article.

What is a datum target? Datum target definition

Datum targets are set of points, line or area required to establish datum on irregular surfaces.

We all know that irregular surfaces are difficult to measure as they can not be placed properly on any fixtures. So it is essential to establish a virtual datum that can come in contact with the surface of the fixture for measurement. Datum targets create those virtual datums.

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Why datum targets are required?

Manufacturing operations like molding, casting, forging has irregular surfaces. If you place those parts on the fixture, the part will never sit on the surface, and it may wobble. Due to this, there is a high chance of measurement error.

So instead of declaring a whole surface as a datum, which is not possible for irregular surfaces, the engineer shows data points that should contact the data feature simulator—a fixture or measuring equipment to establish simulated datums.

To create datums from datum targets, there should be a minimum of three datum points on a surface. Those datum points should not be in a line. When we connect those datum points, we get the virtual datum.

When there are three touch points then it is a primary datum, for two points it is a secondary datum and for one touchpoint it is the tertiary datum.

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Why datum targets use basic dimension?

First of all, we need to understand that data points are virtual or imaginary points to establish the datum to sit on the fixture. Datum points do not play any rule in part form, fit, and function directly. However, it may affect indirectly.

Since there is no direct effect, the engineer chooses not to give any tolerance as the fixture tolerance or inspection equipment tolerance will cover up for that. That is the reason all datum targets use basic dimensions.

Datum Target Symbol

datum target

Types of datum targets

Here are few types of datum targets shown in engineering drawings.

  • Point datum target
  • Circular datum target
  • Rectangular datum target

Point Datum Target

The point datum target shows datum point on the irregular surfaces, which need to be used to establish as a datum. All three points can not be in a single line. Dimensions are placed for all those datum points ( X & Y). All those dimensions are basic dimensions without any tolerances.

In this example you noticed that datum target A1,A2 and A3 is used to to create datum A.

datum target

Circular datum target

Sometimes using only points can be difficult to create a datum because it is tough for a fixture to touch a specific point. Also, it increases the inspection cost.

Due to that reason, the drawing shows a circular datum target zone which is called a circular datum target. So the fixture now has an area or zone to touch. The fixture can touch any point in the zone, or it can touch the whole zone. This gives inspection engineer a bit of roaming to establish the datum.

datum target

Here in this image, you can notice that three circular datum targets ( A1, A2, and A3 ) are used to create datum A. The upper part of the circular symbol shows the diameter of the circular zone, and the lower part shows the sequence number

Rectangular Datum Target

Like circular , many engineer uses a rectangular datum target zone to establish a zone. This is called rectangular datum targets.

In the rectangular datum target symbol, the upper half shows the rectangle’s dimension (length and width ), and the lower half shows the datum target sequence.

datum target

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Advantages of Datum Target

  • Create datums on irregular surfaces
  • Makes inspection easy by establishing datums
  • Lower chance of part rejection
  • The better quality part is possible.
  • Fixture design becomes easier.

Disadvantages of datum target

  • Increases inspection cost
  • Special types of equipment are required for inspection
  • A skilled technician is required.


I hope we all got a good idea about the datum target. Right? As I already said, if you are working in the molding, casting, or forging domain, you are likely to come across datum targets very frequently. Even in other fields, also using datum target becoming very popular. So having a good idea about the datum target is essential.

I tried to cover this topic as best as I can. But still, if you have doubts, please do write in the comment section, and I will be happy to help.

Frequently Asked Questions (FAQ)

What is a datum target?

Datum target a set of point, line or zone used to create virtual datum on irregular surfaces.

What dimension is placed in the lower and upper half of the datum target symbol?

The lower half shows the datum target sequence number and the upper half shows the dimension of the zone ( Circular or rectangular)

Why are datum target dimension basic?

Datum targets are virtual points, and tolerance of fixtures, simulator, or measuring equipment takes care of any deviation in the dimension of those datum targets. That is why engineering drawing shows those dimensions as basic.

Can the diameter symbol be used when the datum target area is round?


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