Oh boy, what a stupid idea – a scientific approach to a DooM-related problem. And that problem is… Linear/Nonlinear maps: which is which?
Poster with all LPD's for UD, D2 and Final DooM https://www.dropbox.com/s/79ftcxzms6nppzr/DooM-LPD.pdf?dl=0   Since nobody actually tried to make such a classification and analyze maps based on it (or did he? Couldn’t find it) I've developed my own. I just didn't talk about it with anyone yet, so I hope it'll be interesting to discuss. And, off course, not everything here is 100% definite – those are simply my suggestions based on study and observation of various maps and are subjects of discussion and corrective actions:).   First of all, this classification does NOT say anything about pros and cons of either linear or  nonlinear maps, it only tries to generalize and simplify their classification. It also ignores such map properties as setting, difficulty, size, length, appearance (indoor/outdoor), shape (flat/volumetric), overall quality etc.   So, there are some basic terms and rules regarding map classification by Level Progress and interpretation of maps based on it using Linear Diagrams which I call Level Progress Diagrams.   Level Progress Diagram (LPD) is a simplified representation of map that depicts its Level Progress. LPD may be presented in either detailed or simplified to any extent forms.   Level Progress is a sequence of all mandatory actions performed by player from the start of map till exit. These actions include going thru Pathways and visiting Key Points.   Pathway is a map area that connects two keypoints. It can have any shape, for example that of a narrow corridor, small room or giant arena.   Key Point is a map area that is mandatory to visit in order to progress further because it contains an essential object (key, switch, boss, locked door).   Depending on the type of Level Progress all maps can be divided into Linear and Nonlinear.   *LINEAR map provides level progress that can be simplified into a line on LPD. Examples of typical linear maps include: E1M8, E2M1, E3M1   Linear maps not always appear in their pure form, in that case they are classified as Generally Linear.

Generally Linear maps may include structures of various nonlinear types (see further) but they are minor and don’t affect gameplay that much. Example: E1M2 is generally linear, even though it has two staircases to red key room the rest of a map is strictly linear.   *NONLINEAR map contains alternative paths to key points, it’s level progress remains different from simple line after simplification on LPD.   Nonlinear maps can be divided into 3 general types (which also may be combined):

I) Fork maps generally include 2 or more alternative paths to keypoints (open or secret) which are significantly different from each other. This includes different ways to key/switch room, different ways to a key door, different paths with a same key color door at the end which lead to same area.
Example: E1M1 is typical fork level since it has a secret path from beginning almost till the exit of map.   There are also special types of forks, not so common: Key optional map has a key door and alternative paths without locked door that are either harder or hidden in the secret. Key interdependent map is a second grade key optional type, it provides different ways to obtain keys + all of them may be obtained in any order. Example: Alpha Accident E1M2 is typical a key optional level, you can pickup blue key via red door or go longer way thru the cellars. Also, an area with a switch can be accessed either thru red or blue key door.   II) Collector maps have multiple independent ways to key points which may be visited in any order. Example: E2M6 is a typical Collector level, you hunt 3 keys in different areas the way you want,  exit door is behind 3 colored doors.   III) Grid maps have multiple ways to keypoints as well as multiple interconnections between pathways. Example: E3M2 is a typical Grid level since you have to navigate thru open maze of rocks to collect blue key   Sandbox is a spacious type of grid map, it has more intersections and they are wider and bigger than keypoint areas, so that pathways are less defined than in a grid level. Example: E3M6 is a typical outdoor Sandbox level.   Those  3 types and their subtypes may be are combined or even be included in a generally linear map. Examples: E4M3 – a collector sandbox with a fork and key optional areas. E4M5 – key optional grid E4M1 – generally linear with little fork E1M7 – generally linear with grid in the beginning   Level progress diagrams also may have special pathway indications – Arena, One-Way Path and One-Side Access. Complete simplification of LPD may remove them sometimes (but not always).   Arena is a large map area with minor structures or obstacles inside. It may belong  to either linear or nonlinear map. Arena without any obstacles inside may count a linear map structure (since it becomes simply a big room). Arena with lots of obstacles (such as columns, walls, ledges, fences, stairs, ramps) that lead to keypoints in significantly different ways is nonlinear map structure.   One-Way Path indicator is put if after achieving a key point player can’t go back the same way.  It’s put for convenience of non simplified LPDs, after complete simplification they become simple lines. Example: E3M3 has a one-way path from flesh ceilings into blood channels   One-Side Access differs from One-Way Path, because it removes itself after being activated. Example: in D2 Map28 player can access yellow key from the throne room, also there’s a way back to yellow door thru cavern with masterminds. If he gets to yellow key from the other side he needs to return back to yellow door thru the same corridor.
LPDs for all Ultimate DooM and DooM2 maps:     Overall, here is my final verdict for all Ultimate DooM and DooM2 maps based on LP classification. E1M1: nonlinear E1M2: linear E1M3: nonlinear E1M4: nonlinear E1M5: linear E1M6: nonlinear E1M7: linear E1M8: linear E1M9: nonlinear E2M1: linear E2M2: nonlinear E2M3: linear E2M4: linear E2M5: nonlinear E2M6: nonlinear E2M7: linear E2M8: arena E2M9: linear E3M1: linear E3M2: nonlinear E3M3: nonlinear E3M4: linear E3M5: nonlinear E3M6: nonlinear E3M7: nonlinear E3M8: arena E3M9: linear E4M1: linear E4M2: linear E4M3: nonlinear E4M4: nonlinear E4M5: nonlinear E4M6: linear E4M7: nonlinear E4M8: nonlinear E4M9: nonlinear   Map01: linear Map02: linear Map03: linear Map04: linear Map05: nonlinear Map06: linear Map07: linear Map08: linear Map09: nonlinear Map10: nonlinear Map11: nonlinear Map12: nonlinear Map13: nonlinear Map14: nonlinear Map15: nonlinear Map16: nonlinear Map17: linear Map18: linear Map19: nonlinear Map20: nonlinear Map21: linear Map22: linear Map23: linear Map24: nonlinear Map25: linear Map26: linear Map27: linear Map28: nonlinear Map29: linear Map30: linear Map31: linear Map32: linear