Technical descriptions and corner coordinates

ONE WAY, NOT THE ONLY WAY

QGIS usually offers several ways to do the same task: a processing algorithm, the Field Calculator, a plugin, or editing by hand. The steps in this module are examples of how to do each task, tested with the course’s sample data. They are not the only way, and other methods are equally valid as long as the outputs meet the guide’s requirements and agree with each other. Where a choice affects correctness (for example, using area($geometry) instead of $area), the module says why.

Before we begin

NOTE

For exercises in this module, you will use the bnhr_qgis-sflma-mapping-001.gpkg inside the data subfolder in the BNHR_QGIS-for-SFLMA-Mapping folder, and the area_corners and area_boundary shapefiles from Preparing the area data.

What we want to achieve

1. Number the corners of the application area consistently.
2. Compute the northing and easting of every corner with Add X/Y fields, as the guide prescribes.
3. Generate the technical description in the standard bearing-distance format, including the tie line.
4. Export the technical description and the computed coordinates with the prescribed names.

What we have

1. area_boundary - the boundary of the sample application, with its attributes filled in
2. area_corners - the empty corners shapefile with the prescribed fields
3. sample_tie_point - the tie point of the sample application and its coordinates

Relevant QGIS knowledge/skills

• Field calculator
• Expressions
• Selects and filters by attribute
• The QGIS Processing Framework
• Exporting layers from QGIS

Anatomy of a technical description

LEARNING OBJECTIVES

1. Identify the parts of a technical description in the bearing-distance format.
2. Differentiate a bearing from an azimuth.

The guide requires the technical description to be presented in bearing-distance format, and the coordinates of the tie point and the description of the tie line must always be given and complete.

A technical description has three parts:

Part	Contents	Example from the sample application
Lines	One row per side of the area: the line (from corner to corner), its bearing, and its distance in meters	1-2 · S 70° 48' 03" E · 186.90
Tie line	The bearing and distance from the tie point to corner 1	Point 1 is N 35° 19' 55" E, 1,283.45 meters from BLLM No. 1
Original coordinate of the point	The reference point’s name, easting, northing, and datum and zone	Easting 560167.218, Northing 1628537.085, PRS 1992 Philippines Zone III

Bearings and azimuths

An azimuth is measured clockwise from north, from 0° to 360°. A bearing is measured from north or south, toward east or west, from 0° to 90°. QGIS computes azimuths, so every bearing in this module is converted from an azimuth.

Azimuth	Bearing
0° to 90°	N azimuth E
90° to 180°	S (180° − azimuth) E
180° to 270°	S (azimuth − 180°) W
270° to 360°	N (360° − azimuth) W

One example line in each quadrant, with its azimuth and its bearing

Numbering the corners

LEARNING OBJECTIVES

1. Extract the corners of the application area from the polygon.
2. Number the corners consistently with the technical description.

3.1. Extracting and numbering the corners

Level: Beginner

1. Run Processing Toolbox ‣ Extract vertices on area_boundary.

Extract vertices on the area boundary

2. The output has ten points for nine corners: in a closed polygon, the last vertex repeats the first. Select it with Select by Expression () and delete it:

"vertex_index" = maximum("vertex_index")

The closing vertex is the one with the highest vertex_index

3. Add T_CORNER to the vertices with the Field Calculator (), as a Whole number (integer) field. A polygon plotted from a technical description, as in Exercise 2.2, starts at corner 1, so:

"vertex_index" + 1

Corner numbers from the vertex index

If corner 1 is another vertex (for example, a polygon digitized from a different starting point), note that vertex’s vertex_index and number the corners from it. For corner 1 at vertex index 3:

(("vertex_index" - 3 + count("vertex_index")) % count("vertex_index")) + 1

4. Keep the Vertices layer: the next exercise computes the corner coordinates on it and then copies the corners into area_corners.

SOMETHING TO THINK ABOUT

If the technical description numbers the corners clockwise and the polygon stores them counter-clockwise, what happens to every bearing in the table?

Computing the corner coordinates

LEARNING OBJECTIVES

1. Compute the northing and easting of every corner with Add X/Y fields, as the guide prescribes.
2. Know when Add X/Y fields is safe to use.
3. Confirm that the computed coordinates match the shapefile.

The guide says: “The preparer shall calculate the Northings and Easting (NE) coordinates of the area using the Add X, y coordinates function in GIS. The calculated NE in shapefiles should be matched to ensure the correctness of the location of the area being applied for.” In QGIS, that function is Processing Toolbox ‣ Add X/Y fields to layer.

When is Add X/Y fields safe?

Add X/Y fields computes correct coordinates, but two of its defaults work against the prescribed shapefile. It is safe when:

• The coordinate system is the layer’s own CRS, the datum and zone of the technical description. The tool’s default is EPSG:4326, which writes degrees: corner 1 of the sample becomes 121.567012 and 14.733692. Any other CRS is converted silently: with Luzon 1911 Zone III instead of PRS 1992 Zone III, corner 1 moves by 7.7 meters without a warning.
• It runs normally, not with Edit Features In-Place. In place, the tool stops with Destination field not found.
• Its fields stay out of the submission. The tool adds new fields instead of filling EASTINGS and NORTHINGS, and on a layer that already has the prescribed X and Y it names them x_2 and y_2. Use a prefix such as NE_ so the new fields are clearly yours, and run the tool on the vertices rather than on the prescribed shapefile. Copying the corners into the shapefile afterwards keeps only the prescribed fields.
• The values are rounded. The tool keeps ten decimals; the guide’s coordinates have three.

3.2. Computing NORTHINGS and EASTINGS with Add X/Y fields

Level: Beginner

1. Open Processing Toolbox ‣ Add X/Y fields to layer and choose the numbered Vertices as the input. Note the default Coordinate system:

The default coordinate system is EPSG:4326 (WGS 84): it would write degrees

2. Set the Coordinate system to EPSG:3123 - PRS92 / Philippines zone 3 (the layer’s own CRS) and the Field prefix to NE_, then click Run.

Add X/Y fields in the national grid, with the prefix NE_

3. In the output layer (Added fields), add EASTINGS and NORTHINGS with the Field Calculator as Decimal number (real) fields of length 20 and precision 3, rounding the new fields to the millimeter:

round("NE_x", 3)

round("NE_y", 3)

EASTINGS from NE_x, to the millimeter

The added fields NE_x and NE_y, and EASTINGS and NORTHINGS computed from them

4. Select all the features of Added fields, copy them (Edit ‣ Copy Features), select area_corners, toggle editing, and paste them (Edit ‣ Paste Features). QGIS fills the fields that have the same name: T_CORNER, EASTINGS, and NORTHINGS, and also NAME, TENURE, REGION, PROVINCE, MUNI_CTY, and BARANGAY, which the vertices carried over from the area boundary. NE_x, NE_y, and the other helper fields do not exist in the shapefile and are left out, and X and Y stay blank. Save your edits.

The corners with NORTHINGS and EASTINGS filled in, and X and Y left blank (NULL)

5. Do the guide’s check: the computed coordinates must match the shapefile. On area_corners, Select by Expression () the corners whose NORTHINGS or EASTINGS differ from their position by more than a millimeter. No corner should be selected.

abs("EASTINGS" - x($geometry)) > 0.001 OR abs("NORTHINGS" - y($geometry)) > 0.001

The guide’s check: no corner’s NORTHINGS or EASTINGS differ from its position

THE SAME VALUES WITH THE FIELD CALCULATOR

For a layer that already has its corners, updating EASTINGS and NORTHINGS directly in the Field Calculator gives exactly the same values as Add X/Y fields with the layer’s own CRS:

round(x($geometry), 3)

round(y($geometry), 3)

EASTINGS from the X coordinate of each corner

If the corners did not come from the area boundary, fill their shared attributes from it with aggregate('area_boundary', 'max', "NAME"), changing the field name for each attribute.

EASTING IS X, NORTHING IS Y

Easting and northing are easy to swap. In the national grid, eastings are around 500,000 m near the central meridian, while northings in the Philippines are in the hundreds of thousands to millions of meters. If your easting is the larger number, check whether the two were swapped.

The nine numbered corners, clockwise from corner 1

Computing bearings and distances

LEARNING OBJECTIVES

1. Create one line per side of the application area.
2. Compute each line’s bearing in degrees, minutes, and seconds.
3. Compute each line’s distance in meters.

3.3. Generating the lines of the technical description

Level: Intermediate

What we want to achieve

• A table with one row per line of the technical description: the line label, bearing, and distance.

1. Convert the area boundary to lines with Processing Toolbox ‣ Polygons to lines.
2. Split the boundary into one feature per side with Processing Toolbox ‣ Explode lines.

Explode lines: one feature per side of the boundary

3. Save the exploded lines before adding fields: right-click the layer ‣ Make Permanent, and save it as td_lines in a GeoPackage (e.g. bnhr_qgis-sflma-mapping-outputs.gpkg).

TEMPORARY LAYERS CUT TEXT AT 10 CHARACTERS

In a temporary (memory) layer, a new Text (string) field from the Field Calculator can hold only 10 characters, and longer values are cut without warning: S 70° 48' 03" E becomes S 70° 48' . Save the layer to a GeoPackage first; GeoPackage text fields have no length limit.

4. With the Field Calculator, add these fields to td_lines, in order:

SEQ (Whole number), the number of the corner each line starts from, used to sort the lines:

aggregate('area_corners', 'max', "T_CORNER", filter:=distance($geometry, start_point(geometry(@parent))) < 0.001)

LINE (Text), the corner numbers at both ends of the line, such as 1-2:

with_variable('a', aggregate('area_corners', 'max', "T_CORNER", filter:=distance($geometry, start_point(geometry(@parent))) < 0.001),
with_variable('b', aggregate('area_corners', 'max', "T_CORNER", filter:=distance($geometry, end_point(geometry(@parent))) < 0.001),
@a || '-' || @b))

LINE from the corner numbers at both ends of each line

AZIMUTH (Decimal number), the grid azimuth of the line in degrees:

degrees(azimuth(start_point($geometry), end_point($geometry)))

The azimuth of line 1-2: 109.199°

BEARING (Text), the azimuth as a quadrant bearing rounded to the second:

with_variable('az', "AZIMUTH",
with_variable('q', CASE WHEN @az < 90 THEN @az WHEN @az < 180 THEN 180 - @az WHEN @az < 270 THEN @az - 180 ELSE 360 - @az END,
with_variable('s', round(@q * 3600),
CASE WHEN @az < 90 OR @az >= 270 THEN 'N' ELSE 'S' END
|| ' ' || floor(@s / 3600) || '° '
|| lpad(floor((@s % 3600) / 60), 2, '0') || ''' '
|| lpad(@s % 60, 2, '0') || '" '
|| CASE WHEN @az < 180 THEN 'E' ELSE 'W' END)))

The bearing of line 1-2: S 70° 48’ 03” E

PRO-TIP

The expression rounds the total seconds first and then splits them into degrees, minutes, and seconds, so 59.6 seconds becomes the next minute instead of “60 seconds”. Save it in the Expression builder (Save expression button) so you can reuse it for the tie line.

DISTANCE (Decimal number, precision 2), the grid distance in meters:

round(length($geometry), 2)

The distance of line 1-2: 186.90 m

The technical description of the sample application

THE CLOSING LINE ABSORBS THE MISCLOSURE

The sample’s boundary was plotted from its technical description in Exercise 2.2, and the generated lines 1-2 to 8-9 match the original exactly. Line 9-1 comes out as N 59° 18' 57" E instead of the original N 59° 19' 00" E: when the closing line was removed, the polygon closed itself, and that last side absorbed the 4.9 mm misclosure. If the application already has a surveyed technical description, submit that one; generate the technical description from the polygon when the polygon is the source.

The tie line

LEARNING OBJECTIVES

1. Compute the bearing and distance from the tie point to corner 1.
2. Write the tie line description in the prescribed format.

3.4. Computing the tie line

Level: Intermediate

What we have

• sample_tie_point - the reference point, with its name and coordinates

1. Save sample_tie_point as tie_point in the same GeoPackage as td_lines (Export ‣ Save Features As…), so that you can add text fields to it.
2. With the Field Calculator, add these fields to tie_point:

AZIMUTH (Decimal number), from the tie point to corner 1:

degrees(azimuth($geometry, geometry(get_feature('area_corners', 'T_CORNER', 1))))

The azimuth from the tie point to corner 1: 35.332°

TIE_BEARING (Text): the same bearing expression as in Exercise 3.3.

TIE_DISTANCE (Decimal number, precision 2):

round(distance($geometry, geometry(get_feature('area_corners', 'T_CORNER', 1))), 2)

TIE_LINE (Text), the tie line in the prescribed wording:

'Point 1 is ' || "TIE_BEARING" || ', ' || format_number("TIE_DISTANCE", 2) || ' meters from ' || "ref_point"

Point 1 is N 35° 19’ 55” E, 1,283.45 meters from BLLM No. 1, Cad-000 (training sample)

The tie line from the tie point to corner 1

IMPORTANT!

The tie point’s coordinates must be in the same datum and zone as the application area. See Don’t convert between Luzon 1911 and PRS 1992.

Exporting the technical description and coordinates

LEARNING OBJECTIVES

1. Export the technical description and corner coordinates in the prescribed formats and with the prescribed names.
2. Prepare multi-parcel technical descriptions in separate sheets.

3.5. Exporting the technical description and computed coordinates

Level: Beginner

1. Put the lines in order with Processing Toolbox ‣ Order by expression, ordering td_lines by SEQ (sorting the attribute table does not change the order features are exported in). Then right-click the ordered layer ‣ Export ‣ Save Features As…:
   • Format: MS Office Open XML spreadsheet [XLSX]
   • File name: TD_IV-A_SFLMA_Bagong Gubat Agroforestry Cooperative.xlsx
   • Layer name: TD (this becomes the sheet name)
   • Under Select fields to export, keep only LINE, BEARING, and DISTANCE

Exporting LINE, BEARING, and DISTANCE to the TD sheet

2. Export tie_point to the same file with the layer name Tie line, keeping ref_point, easting, northing, datum, TIE_BEARING, and TIE_DISTANCE. QGIS asks whether to add a new layer to the existing file: choose to add it, and the spreadsheet gets a second sheet.
3. Order area_corners by T_CORNER in the same way and export it as Comma Separated Value [CSV], keeping T_CORNER, NORTHINGS, and EASTINGS, named TD_IV-A_SFLMA_Bagong Gubat Agroforestry Cooperative.csv.
4. For multiple parcels, export each parcel’s technical description and coordinates to a separate sheet of the same spreadsheet.

QGIS CANNOT WRITE .XLS

QGIS can write .xlsx and .ods spreadsheets but not the older .xls format. If your reviewing office requires .xls specifically, open the .xlsx in a spreadsheet application and use Save As ‣ Excel 97-2003 (.xls).

DISTANCES IN THE SPREADSHEET

DISTANCE is exported as a number, so a spreadsheet shows 186.90 as 186.9. Format the column with two decimals in the spreadsheet if your office expects them.

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