比較的: find the comparison standard in context
Read a 比較的 evaluation with its comparison standard. Relatively flat does not mean perfectly flat or supply a measured slope angle.
A relative evaluation
比較的, read ひかくてき, means relatively. この道は比較的平らで歩きやすい。 says that this road is relatively flat and easy to walk on.
比較的 modifies the evaluation 平ら. The sentence provides no numerical slope angle or height difference for the road.
Check an unstated comparison standard
Even when the comparison is not named in the sentence, context may indicate other roads or typical road conditions. Do not invent a particular road name or measurement.
An evaluation against uneven roads can use a different standard from one against flat roads. Relatively alone does not provide an absolute standard for every road.
Distinguish an evaluation from a measurement
Replacing 比較的平ら with a slope angle of zero degrees makes a stronger claim than the original. Relatively flat and perfectly level are different claims.
If actual dimensions are needed, check a separate measurement. An evaluation modified by 比較的 is not itself a numerical measurement result.
Must a road described as 比較的平ら have a slope angle of zero degrees?
Show the answer and why
No. The description is an evaluation against a comparison standard. It gives no measured angle or guarantee that the road is perfectly level.