# Methods and Applications of Space Understanding in Indoor Environment--A Decade Survey
## 2 Review Methodology and Conduction
### Review Protocol  
All requirements and data sources were combined into a unified review protocol. We specified two flows of processing: main flow and survey extension. The first one determined and processed articles returned from the initial search. The second one focused on 2D image processing and extended the survey with potentially relevant articles found as referenced in the initial set and described as highly important. The entire procedure we proposed consisted of twelve steps: ten of the main flow and two of the survey extension. Each protocol step was done sequentially and extended with notes about its execution. A summary of the articles gathering is presented in the diagram in Figure 1. We represented every single step of the protocol with one dedicated block of the diagram. Each block comprises the step number, its name, and the filtering criteria used (if any were applied). Red numbers are dedicated to the main survey flow. Blue numbers indicate the survey extension process and the number of additionally reviewed papers. Details of each step's execution are described in the following part of this section. **STEP 1: Initial search. ** The combined search phrase was used to query all six proposed databases. The number of returned results was noted separately for each database. Without a filter for publication year, we found over three thousand records. Most were found in the Springer Link database, and the least in IEEE Explore. **STEP 2: Attributes filtering. ** Depending on the actual features of the database search engine, the article's publication date (EC1) was manually checked or introduced additional parameters to the search query. Applying the first exclusion criteria reduced the number of overall records returned by over 35%. The reduction ranged from 22% for IEEE Explore to 45% for Springer Link. **STEP 3: Results concatenation. ** All papers found were aggregated to form one larger initial set of articles for further filtering. The entire set was checked, and duplicated findings were removed (EC2). The concatenated results constructed a collection of over two thousand records. Duplicate removal excluded 236 papers from further analysis, leaving 1854 records in the set. **STEP 4: Titles screening. ** The first level of actual paper reading was to examine its title. Initially, it was checked for the language used (EC3). Later, the title's topic was assessed for conformity to the room segmentation and classification problem (EC4) andindoor environment usage (IC1). Over two-thirds of the papers were removed, leaving 634 records in the set. **STEP 5: Abstracts screening. ** Abstracts were assessed and checked for language (EC3) and topic (EC4, IC1). Additionally, we confirmed that the proposed methodology worked without extended user involvement (IC2). Abstract screening reduced the set of papers by 36%, leaving 402 records in the collection. **STEP 6: Full-text attainment. ** If all previous criteria were satisfied, the publication was checked for access availability (EC5). The whole paper had to be downloaded as a file or easily readable in an online reader. **STEP 7: Full-text screening. ** We checked the publication the same way as titles and abstracts (EC3, EC4, IC1, IC2), but this time with the use of the whole article's text. The number of filtered records was noted for steps 6 and 7 combined, reducing the total set by an additional 57%, leaving 172 papers in the collection. **STEP 8: Bucketing by input data type. ** In this step, we answered the first research question (RQ1). For each article, we specified the processed input data type and used it to categorize the papers. The analysis led to the creation of five buckets of publications. Selected for further analysis, the bucket of 2D images turned out to be the largest one. Including the survey extension, we assigned 87 articles to it. We found a similar number of papers discussing 3D spatial data. The rest of the discovered data types were significantly less numerous.