Aerial surveys aren't a replacement for ground surveys even with the modern technology available today, including GPS, ground scanning radar, photographic equipment. You still need to send surveyors on foot for precision, and to see obstacles that are not apparent from the air. And you need ground surveys to tell you about the nature of the ground, for example, how hard will it be to dig through this rock, will this loose sandy soil cost more money to shore up, stuff like that.

An air survey from a balloon back in those days could have given a general idea of the topography, but this was already well known long before the French started building their canal. There were several available paths known to the French, and the the decision about which to pick depended on ground data of the kind I mentioned above.

France didn't choose "the wrong path", they chose the right path, but it was the wrong kind of canal.

Ferdinand de Lesseps was in charge of the French canal. He had just built the Suez canal, which is a sea-level canal, so he planned to make the Panama canal sea-level too. The French route was optimized for a sea-level canal, and it was pretty good for that purpose. But the problem was that the surveys had been done in the dry season, and de Lesseps didn't realize how badly the Chagres river floods in the rainy season. Flooding would make the canal un-navigable during the rainy season.

Of course the French realized this quickly enough once they started building. But at that time the only option was to divert the Chagres river to avoid flooding the canal zone, and this would be too costly. Another problem was the massive death toll among his workers, in the middle of the jungle and beset by dengue and malaria. It became very hard to find labor.

The Americans took advantage of the French experience and knew before they even started that a sea-level canal wouldn't work. So they decided to elevate the canal about 90 feet above sea level and build locks at both ends. Of course this required a different route, since now they would be avoiding some of the low lying areas on the French path. This isn't because the French path was wrong and theirs was right, it was because they were building two completely different types of canals. A route that would be optimum for one isn't optimum for the other. Still, they reused a lot of the French engineering already done on the higher elevations along the route.

A higher canal required a lot more excavation than a lower one, which is why the French had initially chosen the lower path. About 170 million cubic yards more, in fact. But once they knew about the flooding problem, the Americans had no other choice.

Another big difference between the French and American ventures was that in those couple decades, Ronald Ross discovered the connection between mosquitoes and malaria/dengue fever. So the American effort included a huge sanitation team, whose sole job was to fill up or cover ponds and ditches near the worksite where mosquitoes lay eggs, fumigate the housing where workers lived, spread oil on open water to kill mosquito larvae, etc. This dramatically lowered the death toll.

We know that the possibility of using observation balloons in Panama was considered: the idea was floated at the vary start of the process, in 1876, by a geographical society in Paris. Once we consider the technology available at the time, and the various problems associated with it, however, it's not difficult to see why the idea was not proceeded with.

Aerial photography

The possibility of taking photos from the area was certainly recognised in this period, and numerous experiments were made. The earliest known aerial images shot from a balloon were taken by Gaspard-Félix Tournachon (better known by his pseudonym, Nadar) in France in 1858, from heights of between 260 and 1600 feet, and balloons were in use for surveying purposes in France as early as the 1840s. The first book on the subject, E.G.D. Deville's Photographic Surveying, appeared in 1889

However, the technical difficulties of making successful exposures in the air remained considerable; overcoming the tendency of the gondola to rotate during exposure, blurring the resulting image, was a particular problem. In consequence, as Mattinson concludes, "aerial photography in the 19th century from anything other than artificial, fixed structures or geographic features was more of a novelty than a reality."

Surveying from a balloon

In addition to the above problem, the difficulty of conducting a worthwhile survey from a balloon in areas where the ground was not actually visible was very considerable. Seen from the air, possible canal routes looked like nothing more than undifferentiated jungle. The underlying topographical features of the land beneath the forest canopy was extremely hard to discern, and single exposures made by single cameras were all but useless for revealing them; stereoscopic images were required. A US army officer, C.B. Adams, did receive a patent in 1890 for a set of camera apparatus that could be mounted in pairs of balloons in order to produce overlapping photos that could be converted into topographical maps, but his invention was never commercialised and the problem of flying balloons in pairs in Panama would have doubled the other difficulties outlined here.

Type of balloon

Had a decision been taken to deploy balloons in Panama, there would have been broadly two ways in which it could have been done. Prevailing winds in the isthmus blow to the north-east for most of the year, so it might have been possible to release a balloon on the Pacific coast and let it drift across the country to the eastern coast. However, there would have been two major problems in practice. One is that, while the prevailing winds blew north east, the canal follows a route that runs from the north-west to the south-east of the country. In other words, a free balloon could not be flown along the planned route in any but the most exceptional weather conditions.

Secondly, and still more problematically, the isthmus of Panama is only 50 miles across, and the planned canal route was heavily covered in jungle almost throughout. Putting a balloon down on a narrow strip of the Caribbean coast would have been an extremely difficult and dangerous thing to attempt; failure to get it right would mean either crashing in the rainforest, or being blown out to sea, either of which would very likely have proved fatal.

The obvious alternative would have been to use tethered balloons, but this, in turn, would have required the construction of roads through the jungle – one for each possible or planned route – that were wide and flat enough to allow a balloon and all the necessary men and ancillary equipment required to fly it to be taken out into the middle of the isthmus. The physical and medical problems this would have entailed would have been enormous.

Either method would also have required the surveyors to overcome the problem of inflating their balloon(s). By the time the canal was constructed, hot air ballooning had been superseded by gas ballooning, and hydrogen balloons – which produce significantly greater lift – were common. Nonetheless, shipping equipment capable of generating or storing the necessary quantities of hydrogen out into the jungle would have been another logistical nightmare.

Disease

Finally, aside from the enormous physical effort and cost that would have been required for such an effort, there is one other good reason why such a plan would almost certainly not have worked: disease. As is reasonably well known, the French struggled for decades to construct a canal before the project was taken over by the US. The single most significant reason why the French failed, while the Americans succeeded, was that the French failed to master conditions that were perfect for the spread of malaria and yellow fever, while the US Army Engineering Corps, led by William Gorgas, succeeded. The French lost almost 25,000 workers to disease during their attempts at construction; had they also tried to drive multiple survey roads through the rainforest, that total would undoubtedly have been significantly higher.

Sources

David Mattinson, "Aerial photography," in John Hannavy (ed.), Encyclopedia of Nineteenth-Century Photography (2008)

Matthew Parker, Hell's Gorge: The Battle to Build the Panama Canal (2008)

Henry Petroski, "Engineering: The Panama Canal", American Scientist, Vol. 81, No. 1 (January-February 1993)

Thank you for Both comprehensive answers. I am starting to read D. McCullough "Path Between the Seas" and about the misery and loss of life during the countless surveys which obviously precludes a huge loss of life building along those routes. It seems like a reasonable approach to find a path that would duplicate the creation of an artificial lake to duplicate the features in Nicaragua. As such, in my imagination, an aerial survey need only to concentrate on finding low lying areas and the highest summits on ingress and egress.

I now read that a Frenchman, de Lepinay, advocated for that artificial lake idea at the same Paris convention but was shouted down by de Lesseps because it didn't fit his "plan" of a ditch dug through out the entire path across the isthmus. The Americans must have used de Lepinays plan when they created Lake Gatun