A altitude é baixa; contudo, o desnível é grande.

In Portuguese, it is very common to use the definite article with nouns where English often uses no article.

So:

• a altitude = the altitude
• o desnível = the elevation difference / the unevenness / the drop or climb

Even when English would say something more general like Altitude is low, Portuguese often prefers A altitude é baixa.

This is completely normal and natural in Portuguese.

Baixa changes because it agrees with altitude, which is a feminine singular noun:

• a altitude
• baixa

Grande is a little different: it has the same form for masculine and feminine in the singular.

So:

• o desnível é grande
• a diferença é grande

Both are correct, and grande does not change here.

Portuguese often uses ser for more general or defining characteristics, and estar for temporary states or conditions.

Here:

• A altitude é baixa = the altitude is low
• o desnível é grande = the elevation difference is great

These are being presented as factual characteristics, not temporary conditions, so ser is the natural choice.

Using está here would usually sound odd unless there were a very special context.

This is an important vocabulary point.

• altitude = altitude, height above sea level
• desnível = difference in level, change in elevation, slope difference, vertical gain/loss

So the sentence contrasts two different ideas:

• the place itself may be at a low altitude
• but the route or terrain may involve a large elevation change

This kind of contrast is especially common in hiking, cycling, geography, and route descriptions.

Contudo means however / nevertheless / still.

It is a correct and fairly common linking word, but it is a bit more formal or written than mas.

Comparison:

• A altitude é baixa, mas o desnível é grande. → more everyday
• A altitude é baixa; contudo, o desnível é grande. → more formal/written

So yes, contudo is natural, but it has a slightly more polished tone.

The semicolon links two closely connected independent clauses:

• A altitude é baixa
• contudo, o desnível é grande

It works a bit like stronger punctuation than a comma, showing a clear contrast.

You could also write:

• A altitude é baixa, contudo, o desnível é grande.
• A altitude é baixa. Contudo, o desnível é grande.

All are possible, but the semicolon gives a neat, formal written style.

Yes. Portuguese allows some flexibility.

These are all possible:

• A altitude é baixa; contudo, o desnível é grande.
• A altitude é baixa; o desnível, contudo, é grande.
• A altitude é baixa. Contudo, o desnível é grande.

The first version is the clearest and most neutral for learners.

Yes, absolutely.

• A altitude é baixa, mas o desnível é grande.

This is more conversational and probably the most common everyday version.

The difference is mostly one of style:

• mas = simpler, more common in speech
• contudo = more formal, more written

It is singular because the sentence refers to the elevation difference as one overall idea.

So:

• o desnível é grande = the overall elevation change is large

If you were talking about several separate uneven parts or level differences, then a plural might make sense:

• os desníveis são grandes

But in route descriptions, singular is very common when talking about one route, one climb, or one general vertical difference.

In European Portuguese, desnível is very common in contexts like:

• hiking
• road or trail descriptions
• cycling
• geography
• construction / terrain

It often refers to the difference in height between two points or the amount of climbing/descending.

For example, in hiking you may see something like:

• um percurso com muito desnível
• desnível acumulado
• desnível positivo

So this sentence sounds very natural in that kind of context.

A rough English-friendly guide:

• altitude ≈ al-tee-TOO-d(uh)
• baixa ≈ BY-sha
• contudo ≈ kon-TOO-doo
• desnível ≈ duzh-NEE-vel or desh-NEE-vel depending on accent and speed

A few helpful notes for European Portuguese:

• unstressed vowels are often reduced
• x in baixa sounds like sh
• the stress in desnível is on ní

If you want to sound more natural, focus especially on the stress:

• altiTUde
• BAIxa
• conTUdo
• desNÍvel

Not naturally in this structure.

Portuguese normally keeps the verb in both clauses:

• A altitude é baixa; contudo, o desnível é grande.

Leaving out the second é would sound incomplete.

Unlike in some short English styles, Portuguese usually does not omit the verb here.

Yes, baixa is the normal and natural adjective with altitude.

• altitude baixa = low altitude
• altitude elevada = high altitude

You might also see:

• pouca altitude in some contexts, but this is less standard here
• altitude reduzida, which is more technical or formal

So A altitude é baixa is a very good, normal phrase.

Yes, very often.

Literally, it means the elevation difference is large, but in real-world contexts it often suggests that:

• the route may be physically demanding
• there may be a steep climb or descent
• the terrain is not as easy as the low altitude might make you expect

So the contrast in the sentence is meaningful:

• low altitude does not necessarily mean easy terrain

That is probably the main point of the sentence.