What are the limitations of using projected data?

[nazmussazib1]

Can anybody help me about the limitation of using projected data over geographic coordinate system.

[DanPatterson_Retired]

I can think of a few...but certainly not limited to...

• not being able to do geometric calculations on a global or continental scale simply... you have to resort to complex geometric calculations using spherical coordinates. These would include.
  • determining all the locations on a line formed by too longitude/latitude locations...you definition of the line type yields several options.
  • calculation of area  and perimeter is affected by the above
  • lets not even get into doing anything geometric involving the above
• a lack of a standard projected coordinate system (PCS) given the many different Geographic coordinate systems (GCS) that exist currently and historically
• PCS should have clearly tabulated error statements when calculating geometric properties/calculations with regards to latitudinal issues and differences from things like the central meridians or standard parallels (ie how accurate is area calculation to the extreme east of a utm zone as compared to those centered over the central meridian

Given these very few limited examples, it should be obvious that describing "where are we" is difficult enough, but when you add the other dimensionalities, the issues that need addressing compound drastically.

[nazmussazib1]

Thanks a lot for your answer. As far as i know, all of the projected coordinate system consider earth as sphere, where as earth is not truly sphere. Is there any implication for that in raster processing?

[DanPatterson_Retired]

Not all projected coordinate systems treat Earth as a sphere.  In fact most define an equatorial radius and a polar radius , or an inverse flattening, as evidenced in this UTM Zone 18 spheroid definition:
portion of *.prj extracted ...SPHEROID["GRS_1980",6378137.0,298.257222101]...

All considerations, whether a sphere or spheroid will have an impact on the issues I address, for both raster and vector data.  Due note, the geoid hasn't even been considered.

[MelitaKennedy]

To expand a little, many projections that are designed to display the whole world like Mollweide, various Eckert ones, and so on do have sphere-only algorithms. Projections that support large and medium scale data (including all conformal projections) have ellipsoid and sphere algorithms.