There is one question about spatial technology that bothering me for a couple of years.

Is anyone suitable for working with GIS and spatial data or one should be a professional?

Well that’s a tough question. But in general it depends on conditions.

Lets ask ourselves two more other questions.

- Is anyone suitable to operate PC, printer or database?
- Is anyone suitable to be system administrator in your office?

Obviously it is “yes” for the first question and “no” for the second. Anyone can ride a bicycle but one need license to drive a bike.

So my answer for the given question is – one need to learn a lot to understand how to work with spatial data. Anyone willing to learn theory is suitable to work with spatial data.

Tonight one smart ass (or should I write “cat”?) asked me why in the hell I suggest to calculate distances (when using PostGIS) in the corresponding zone of Pulkovo-42 for Russia (EPSG:28403 for example) when possible? The motivation of the question was based his statement that one will need far shorter SQL-query for distance measurement if the geometry would by casted to geography type and distance would be calculated with EPSG:4326. Like: “Eah, ST_Distance will return meters even for ellipsoidal calculations! Did you know it, dumb ass?! Admit, you suck!”

Well, kind reader, can YOU see the GLOBAL difference between EPSG:28403 and EPSG:4326 (it is not about projection)? The answer is “reference ellipsoid” (or “geodetic datum” or just “datum”). This reference ellipsoid defines the Earth shape model parameters for all the calculations that one would perform with spatial data.

Well, kind reader, if you were (are) a good student then you should understand, that if reference ellipsoid defines the MODEL of the shape, then it also defines the BIAS of this model from the true value at any given location.

The next questions you may ask:

**Q:** Does this bias matters?

**A:** Well, it depends on your goal requirements and the answer will be “NO” or “YES”. So you may ask me here if in doubt.

**Q:** How big is the difference between measurements using different datums?

**A:** Well, it depends 😉 … depends on datums compared as well as locations compared – from centimeters to hundreds of meters.

**Q:** Will 1 meter difference make your day?

**A:** Absolutely! Do you have a land parcel in your possession? Imagine that tomorrow some people from the municipality will came to you and demand to move you fence, because it is build on the adjusted road land parcel. They show you a digital map where your fence is 1 meter on the adjusted parcel. And this shit happened because of 1 meter bias!

#### Instead of conclusion

If you’re a cat – grab a handbook on geodesy and become a human being, or… just keep licking your balls and quit work with spatial data before you’ll ruin something.