I downloaded 2 1/3 arcsecond DEMs from the national map as .img files. First I ran the Mosaic to New Raster tool in Arcmap. The values of the first DEM are 49.0551 to 1111.13. The values for the second DEM are 274.089 to 3505.05. The output is -78.4316 to 3505.05. I have tried changing some of the options as follows.
Pixel type: 32bit float, 32 bit unsigned (this one gave a low of 0)
Mosaic Operator: All the possible options
So then I tried it in ERDAS with mosaic express. I got the same result.
I looked at the histograms, not just the stretch classification to confirm the values.
I used the pseudocolor option in ERDAS to locatate the negatave pixels of which there are many thousands. They are not located at the seam line, rather it is a big valley that is the lowest area of the DEM.
I downloaded the DEM again, but this time as arcgrid. Now it displays the min of -78 instead of 49 and the max is now 1875 instead of 1111. Not sure why the IMG gave different values. I went to google earth to check if this is an error. Turns out that at the very edge of the DEM it does drop below sea level, so it is correct. I still do not understand why the IMG file did not display the -78 and 1875 in the first place. I would not have been worried if the min and max of the mosaic output was the same as the inputs.