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Parallel corpora

A parallel corpus is a corpus that contains a collection of original texts and their translations into other languages. A parallel corpus can be bilingual or multilingual. In Sparv each text collection of one language is treated as a separate corpus which needs to be annotated separately but may be linked to one or more corpora in other languages. The following example demonstrates how to annotate a bilingual (English-Swedish) parallel corpus with automatic sentence-linking and word-linking.

You can download an example mini corpus including a working Makefile here.

Folder structure

Each corpus has its own root directory. The names of the root directories ( mycorpus-en and mycorpus-sv) need to be identical apart from the language ID suffix. If the language in question should be annotated with Sparv its language ID suffix should be a two-letter ISO code (check the language table in the install section for reference). Files that are linked to each other must have the same name (in this example: sparv-intro.xml).

my_parallel_corpus/
    mycorpus-en/
        original/
            xml/
                sparv-intro.xml
    mycorpus-sv/
        original/
            xml/
                sparv-intro.xml
    Makefile

Linking corpora

Sparv includes tools for automatic sentence-linking and word-linking. Sentence-linking is a prerequisite for word-linking, and sentence-linking requires pre-aligned text chunks (e.g. paragraphs or similar). In the example corpus the pre-aligned chunks are called "link". Each link has an ID (attribute n) which corresponds to the link-element with the same ID its linked translation.

If you are using automatic sentence-linking you need to set the variable align_sentences to true and you must specify sent_align_chunk to the element that has been pre-aligned.

If your text is already linked on sentence level you need to have sentlink elements sourrounding the sentences, in order to get automatic word-linking. They should have IDs indicating which sentences belong together (e.g. <sentlink n="0001">). The IDs should be unique for each indata file. Don't forget to set the variable align_sentences = false in your makefile.

If you would like to use automatic word-linking you need to add the attribute wordlink-$(other_lang) to the variables vrt_columns and vrt_columns_annotations in your Makefile. $(other_lang) is the variable for the language ID suffix of the linked language.

The Makefile

For parallel corpora the following variables must be set in addition to the standard variables:

  • parallel_base: the root directory name without language ID suffix (mycorpus)
  • root: the root directory with language ID suffix (mycorpus-$(lang))

For automatic word-linking:

  • align_sentences: true if sentences should be linked, else false (In this case the sentences must be pre-aligned. Word-linking will not work without sentence-alignment.)
  • sent_align_chunk: the pre-aligned chunk which is parent to sentence

You need to include Makefile.langs in your Makefile (after including Makefile.config). By doing this Sparv will know which analysis tool (TreeTagger or FreeLing) to use for which language.

If the languages you are annotating make use of different analysis modes (e.g. FreeLing, TreeTagger or the standard Swedish mode) you need to define language-specific if-clauses in your Makefile in order for the variables to be set to the correct values. Check the example Makefile for more details.

If you have certain attributes that are not used in all the languages in your parallel corpus you need to list these attributes in the null_annotations variable. In that case you also need to set skip_cwb_compression and
skip_cwb_validation to true, otherwise you will not be able to create the Corpus Workbench binaries.

Annotating

Run each corpus separately and define which language should be annotated (lang) and which language the corpus should be linked to (other_lang; Note: this parameter is set automatically in the example corpus!):

make vrt lang=en other_lang=sv
make vrt lang=sv other_lang=en

When creating the corpus Workbench binaries you also need to run make align for both languages (after creating the vrt-files), e.g.:

make align lang=en other_lang=sv
make align lang=sv other_lang=en

Annotation result

This is what the vrt result could look like for a Swedish-English parallel corpus (taken from the example corpus which can be downloaded above):

<text>
<link n="1">
<sentlink n="sparv-intro.01">
<sentence id="sv0248-sv02a2">
Sparv    NN  NN.UTR.SIN.IND.NOM  |sparv| |sparv..nn.1|   01  |01|
är    VB  VB.PRS.AKT  |vara|  |vara..vb.1|    02  |02|
Språkbankens    NN  NN.UTR.SIN.DEF.GEN  |språkbank| |språkbank..nn.1|   03  |03|04|
annoteringsverktyg    NN  NN.NEU.PLU.IND.NOM  |   |   04  |05|06|
...
</sentence>
</sentlink>
</link>
</text>
<text>
<link n="1">
<sentlink n="sparv-intro.01">
<sentence id="en038c2-en0affc">
Sparv    PROPN   NP  sparv   __UNDEF__   01  |01|
is    VERB    VBZ be  __UNDEF__   02  |02|
an    DET DT  a   __UNDEF__   03  |03|
annotation    NOUN    NN  annotation  __UNDEF__   04  |03|
tool    NOUN    NN  tool    __UNDEF__   05  |04|
developed    VERB    VBN develop __UNDEF__   06  |04|
...
</sentence>
</sentlink>
</link>
</text>

The tab-separated columns in the vrt output contain the following information:

word pos msd baseform lemgram linkref wordlink

In this example the texts were manually linked by the <link> elements. The sentlink element was added automatically by the sentence linking module. If the link element contains multiple sentences the sentlink attribute will break down the linked elements into smaller units. The ID in the link and sentlink elements (e.g. n="1", n="sparv-intro.01") indicates which elements belong together across linked texts in different languages. By default the sentlink-ID is prefixed with the name of the indata file (in this case "sparv-intro"). Note that the prefix is not required for automatic word-linking, so if you use manual sentences-linking you will not need to set a prefix.

The last column (wordlink) contains a set of link reference numbers (linkref) referring to tokens from the linked text within the same <link> element. Note that this set may be empty in some cases, meaning that the token was not linked to any token in the other language.

The column lemgram contains only __UNDEF__ values for the English text because lemgram was listed in the null_annotations since this annotation is only available for Swedish. Columns containing only __UNDEF__ values are not present in the XML export.