How can the Simpson Desert be the largest sand dune desert in the world?

Question

In the Wikipedia article for the Simpsons Desert in Australia in the first paragraph it says:

It is the fourth-largest Australian desert, with an area of $\small\mathsf{176,500}$ $\small\mathsf{km^2}$ ($\small\mathsf{68,100}$ sq mi) and is the world's largest sand dune desert.
Simpson Desert - Wikipedia

I know that the Rub' al Khali Desert is the largest sand desert in the world, and as seen in this photo it has sand dunes:

This is the Rub' al Khali. The caption for this picture on the article says "Sand dunes to the east of Liwa Oasis".

Here is another photo of the Rub' al Khali

Is it the case that although the largest sand desert is in southern Arabia and contains sand dunes nevertheless there is another larger "sand dune desert" in Australia? How is this classified? I'm assuming if this is true it means that the Rub' al Khali Desert doesn't fall into the category of a "sand dune desert", whatever that is.

I've seen photos of both and both look very sandy and duney to me.

Answer 1

The linked Wikipedia article has been edited after the OP's question was posted. It now says:

The Simpson Desert is a large area of dry, red sandy plain and dunes in Northern Territory, South Australia and Queensland in central Australia.^1,2 It is the fourth-largest Australian desert, with an area of 176,500 km2 (68,100 sq mi).

The desert is underlain by the Great Artesian Basin, one of the largest inland drainage areas in the world. Water from the basin rises to the surface at numerous natural springs, including Dalhousie Springs, and at bores drilled along stock routes, or during petroleum exploration. As a result of exploitation by such bores, the flow of water to springs has been steadily decreasing in recent years. It is also part of the Lake Eyre basin.

The Simpson Desert is an erg that contains the world's longest parallel sand dunes.³ These north-south oriented dunes are static, held in position by vegetation. They vary in height from 3 metres (9.8 ft) in the west to around 30 metres (98 ft) on the eastern side. The largest dune, Nappanerica or Big Red, is 40 metres (130 ft) in height.⁴

¹Twidale, C.R. (1980). "The Simpson Desert, central Australia". South African Geographical Journal. 65: 3–17.

²"Desert Parks of South Australia" (PDF). Department of Environment and Heritage, South Australia. Archived from the original (PDF) on 29 July 2008. Retrieved 23 October 2008. "Simpson Desert". Britannica Encyclopedia Inc. Retrieved 23 October 2008.

³Madigan, C.T. 1936. The Australian sand-ridge deserts. The Geographical Review 26(2):205-227.

⁴Rachel Dixon (2018). Deserts & Savannahs in Australia. Redback Publishing. p. 23. ISBN 9781925630206.

It seems that the article has been corrected on 13:13, 2 September 2019 by user‎ Casu Marzu per the article's history page

Considering the timing, this is likely to be a direct result of your question. Good work!

Answer 2

The controversy over the largest sand desert is like the controversy over the longest river. Both Nile and Amazon are often quoted, and I have seen claims for the Mississippi. The Nile is favourite. Photographs of deserts can be misleading unless they are photographed from high above (such as from an aircraft or from space) and show the whole area, because photos from the ground only show a very limited area.

Just as with rivers, much depends on how you classify what you are comparing. There are different ways of measuring rivers and no doubt different ways of measuring deserts. The largest desert is the Sahara, and it also has the greatest area of sand sea and dunes. It doesn't qualify as the largest area of dunes or sand sea in this context because the sand seas, though large, are not contiguous but are intersected by areas of mountain desert and stony desert. In some deserts the sand sea is interrupted by areas of semi desert in which there is sparse vegetation. Even dunes support sparse vegetation in some places, which compounds the problem of whether they count as dunes. How much vegetation can you allow and still classify it as dune or sand sea?

Just as the Nile is the favourite option for longest river, Rub al Khali must be favourite for largest contiguous sand dune desert, and Wikipedia may have got it wrong. But to be sure, we need to know how the Simpson desert was measured.