Great post, Tex. I hope I'm not stepping on your toes, but I thought I'd try to integrate a bit of A-10 specific info into the big picture you painted above.
Disclaimer: Not all of what's written below is simulated accurately (or at all) in DCS, so take this as background information only.
Cockpit Altimeter
The cockpit altimeter is normally operated in the RESET/ELECT mode. In the ELECT mode, the altimeter operates as a servo repeater which is electronically driven by CADC signals and displays barometric altitude. The CADC supplied altitude is corrected only for static port installation error.
When aircraft power is not available, or when commanded using the function switch, the cockpit altimeter operates in the STBY/PNEU mode. In the PNEU mode, the altimeter operates independently to display uncorrected barometric altitude.
(Source: T.O. 1A-10C-1, Flight Manual, USAF Series A-10C Aircraft; MIL-PRF-83419E, Performance Specification, Altimeter, Servo Controlled, Automatic Pressure Standby, Type AAU-34)
HUD Altitude Display
In NAV mode and the Air-to-Air sight, LASTE repeats the altitude displayed on the cockpit altimeter within normal tolerances. The altitude can be adjusted along with the cockpit altimeter using the Kollsman knob.
In the weapons delivery modes (CCIP, CCRP, GUNS), the LASTE system provides three other options of altitude source selection via the AHCP ALT SCE switch. The selected altitude source is used for CCIP/CCRP, Maverick, and TDC ranging.
(Source: T.O. 1A-10C-34-1, Non-nuclear Weapons Delivery)
Altitude Source
BARO
The first option is BARO mode. When BARO mode is selected, the system computes a true MSL altitude from the CADC barometric reference altitude that is adjusted by the Kollsman altimeter setting. LASTE captures the field elevation on takeoff roll (as set on the cockpit altimeter, not Steerpoint elevation), and uses the value as the calibration point from which to calculate the corrections.
The resulting geometric altitude is corrected for static port installation errors, dynamic lag, and nonstandard air temperatures. The geometric altitude also contains a bias correction based upon the EGI GPS altitude.
In BARO mode, the calibration point is valid only for the altimeter setting and air mass which existed at takeoff. Changing the Kollsman
altimeter setting may cause a bias error as great as 250 feet in the geometric altitude displayed on the HUD.
Testing is required to confirm recent changes to BARO mode implementation in DCS.
DELTA
The second option is the DELTA mode. When DELTA mode is selected, the system computes a true MSL altitude from the CADC pressure altitude, which is not affected by the Kollsman altimeter setting. The geometric MSL altitude in DELTA mode is also corrected for static port installation errors, dynamic lag, and nonstandard air temperatures. The calibration reference point for DELTA mode is derived from one of two values: the Radar MSL value (Radar altitude plus target elevation) or the EGI GPS MSL value. This calibration point is first automatically captured on takeoff roll using the displayed barometric altitude.
An in-flight DELTA update can be accomplished at any altitude by first depressing the ENT key on the UFC and using the UFC SEL button to choose either the Radar or EGI GPS modes displayed in the center of the HUD screen. These values can also be manually entered into the DELTA CAL submenu if obtained from other aircraft. The altitude corrections will then start from the new reference point when either a DELTA update has been accepted or new data is entered in the DELTA CAL submenu and the value is STORED.
DELTA mode can be used prior to taking an airborne update because it performs an automatic DELTA update on takeoff, and DELTA altitudes are not affected by Kollsman settings.
DELTA mode is not implemented in DCS.
RADAR
The third option is the RADAR mode. When RADAR mode is selected, the system uses the radar altimeter altitude. The radar altitude is valid up to approximately 5,000 feet. Beyond 5,000 feet, the system estimates AGL altitude for a period of time after which the radar altitude is declared invalid. This is a GCAS function and should not be used for performing a DELTA update.
RADAR mode is not implemented in DCS.
(Source: T.O. 1A-10C-34-1, Non-nuclear Weapons Delivery)
Non-Standard Atmosphere Adjustments (NSATMADJ)
In my earlier post I incorrectly stated that the NSATMADJ value, which is viewable in the IFFCC Data Capture Menus, played a part in altimetry. In actuality, the NSATMADJ value is computed and applied only to the target density altitude (TDA) for the purpose of calculating Real-Time Safe Escape (RTSE) cues and the Minimum Range Staple (MRS).
(Source: T.O. 1A-10C-34-1, Non-nuclear Weapons Delivery)
D-Values
The D-value is the difference between the true altitude of a pressure surface and the standard atmosphere altitude of this pressure surface. Like Tex said, pilots normally get the values from the weather shop, but for background there are two methods for deriving the D-value:
Method 1) Compute the D-value using the formula: D-value = True Altitude - Standard Altitude.
Example:
Determine the D-value for a release altitude of 5,000 feet MSL. Use the appropriate constant pressure chart for the flight level, in this case, the 850mb chart (see "850mb.png" below). The standard height for the 850mb level is 4,781 feet MSL (see "Pressure Levels.png" below). Consulting the 850mb chart, the 850mb level is at 1,470 meters (4,882 feet) in the vicinity of Nellis AFB. Thus:
D-value = (4781 - 4882) = -101 feet
D-values are entered into the DELTA CAL menu as its inverse value, so: 101 feet.
Method 2) Use the nomagram to compute estimates of the D-value between heights of standard pressure surfaces, or between surface altimeter setting and the height of a standard surface (See "D-Value.png" below).
Step 1. Determine the altitude of interest (aircraft flight level, for example).
Step 2. Determine the observed or forecast heights (in meters) of standard pressure levels bounding the altitude of interest (an A-10 at 7000 feet would be bound by the 700mb and 850mb surfaces, for example).
Step 3. If the altitude of interest is below the 850mb level, determine the observed or forecast height of the 850mb level (meters) and the observed or forecast surface altimeter setting in inches of Hg.
Step 4. Plot the heights of the pressure surfaces and/or the altimeter setting on the graph. Connect them with a straight line.
Step 5. Locate the point at which the line crosses the altitude of interest, then read straight up the graph to get the D-value in feet.
(Source: AWS/FM-95/001, Improved Altimeter Settings for A-10 Aircraft; AFWA/TN-98/002, Meteorological Techniques)