Did they find large pieces of previously undiscovered land? Islands? Did it not make a difference as we had already used earlier technologies in order to map our entire world?
Hello. First up I am Surveyor not a historian. but I will do my best to remain factual and cite some sources.
GPS changed the way movement of the earth was mapped more than anything else. Although maps were not drastically changed per say, New coordinate systems and projections were developed to better utilize GPS and satellites. In New Zealand (where I studied) all of the legal maps derived their coordinates from system developed in 1949 (New Zealand Geodetic Datum 1949, or NZGD1949 for short). This system was developed by conducting a survey across the whole country using theodolites to measure angles and steal tapes to measure distance. The land itself was already mapped. This system was how things were defined relative to each other. The NZGD1949 survey was a time consuming process (it took years to do the survey) and inaccurate because the distances involved.
In the year 2000 the system was upgraded (NZGD2000). Instead of conducting a survey off the whole country, GPS systems were set up all around the country that are constantly measuring. These GPS units have accuracies of +/- 5cm and can transfer that accuracy to nearby marks and surveys. The GPS systems can be used to map in close to real time how much the whole country is moving and deforming (This is important as NZ is of a fault line and constantly changing). All of this made all of the maps and plans more accurate than the previous system that relied on one relativly inaccurate survey. Another imporant aspect of this is the whole system is dynamic and can change. When the city of Christchurch was hit by an earthquake in 2011 the Land Information New Zealand was able to patch coordinates that had shifted due to the quake.
The change did result in some changed in maps. But these were a the meter level. nothing drastic. And were only enforced when legal land boundries were being changed or being updated because of a dispute. I believe similar changes occured around the globe but I know less about them.
So I do not know whether any large or small bits off land were discover because of satellites. People have been traveling and navigating off the stars for hundreds of years so I would find it unlikely. But the greater accuracy available with GPS resulted in maps changing but at a much smaller (meter level) and these changes were staggered.
The following website has more info about NZGD1949 and NZGD2000 http://www.linz.govt.nz/data/geodetic-system/datums-projections-heights/nzgd2000-frequently-asked-questions
The book 'Surveying for Engineers' (fith edition) by J Uren and W Price is what we used as a text book in Uni goes into lots of detail regarding the development of techniques from Theodolite to GPS.
The satellite didn't make much difference to mapmaking outside polar regions. Aerial photography had been available for more then 50 years, and the world's coastlines and islands had been pretty well mapped for a couple of centuries. Sailors have a very great interest in not running into unexpected land masses, and coastlines are among the easiest things to map. You can move your observation point about quite easily, and you don't have to try to see through hills or even through thick forests. As I often show schoolchildren, a 19th century map of the Iberian Peninsula differs very little from a modern one.
Satellite imagery did make mapping of polar regions much more practical, and GPS allowed much better measurement of the precise heights of mountains.
Piggybacking off this question; did radar and sonar have an effect on cartography as well in their early days?
Not discouraging other responses, but fyi, you can get started in the FAQ
Follow-up question: how accurate was the famous map of India that was elaborately surveyed?
The first decently accurate maps were developed during the 1600s.
Cassinni is credited with the first reasonably accurate map of France. His family dedicated decades to the measurement of France by laborious triangulation of every part of the country.
As noted here, the phenomena you imagine had more to do with the discovery of how to accurately measure Longitude, long before the invention of modern satellites
“Earlier maps had underestimated the distances to other continents and exaggerated the outlines of individual nations. Now global dimensions could be set, with authority, by the celestial spheres. Indeed, King Louis XIV of France, confronted with a revised map of his domain based on accurate longitude measurements, reportedly complained that he was losing more territory to his astronomers than to his enemies.” ― Dava Sobel, Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time
also noted elsewhere regarding use of the moons of Jupiter:
In the meantime, however, the French had made a different use of satellite eclipses. If it was not feasible to make observations from the deck of a moving ship, it was certainly possible to observe the satellites on land. In the 1670s French astronomers, under the leadership of Cassini, began making observations of the satellites in many locations in France. The resulting map of France, finished in 1679 showed that the west coast of France was too far west by an entire degree on existing maps and that similar adjustments had to be made to the Mediterranean coast. It is said that upon seeing this map, King Louis XIV remarked that he was losing more territory to his astronomers than to his enemies.